The Technology Connections video has already been posted, but for a quick written answer:

Cheap microwaves wire that button to mean "run for 2 minutes" which will probably ruin your popcorn. Mid-ranged brands will sense moisture. Higher end brands might have a microphone to listen for delay between pops which is what the popcorn bags instruct people to do.

Popcorn makers tell you to listen for pops manually because it's easier than writing "if your popcorn button sets a time or asks for bag size, don't use it because it's not using any sensors to determine when to stop and will burn your popcorn."

Just because the microwave has a button it doesn't mean the button does the right sequence of steps.

Technology Connections has a good video on it.
https://www.youtube.com/watch?v=Limpr1L8Pss

This is a great explainer on that topic. Turns out some are fine but most of them are just garbage which is why the warning

https://m.youtube.com/watch?v=Limpr1L8Pss

Theres a technology connections video for that https://www.youtube.com/watch?v=Limpr1L8Pss

Steam.

The answer is steam. Some microwaves have a humidity sensor that measures the amount of moisture released by food during cooking. Using that information, the microwave can estimate when the food is done and, in some models, adjust the cooking time automatically. For example, certain microwaves use this technology for sensor-cooking programs designed for potatoes, vegetables, or other foods.

Unfortunately, these features have largely turned into marketing gimmicks. They help sell microwaves more than they necessarily help cook food correctly. Many models no longer include genuine humidity-sensing technology, and the single-food buttons on cheaper models are often nothing more than preset cooking times and power levels.

People buy appliances based on how they look and how well they can show them off to others. Very few consider how a feature actually works, whether it serves a practical purpose, or whether it offers any real benefit over a less expensive appliance.

Here are a few other questionable marketing gimmicks in modern appliances:

Refrigerator air filters: These can reduce certain odors, but their effectiveness varies. The problem is that manufacturers turn them into another recurring expense for the owner, even when their practical benefit is limited.

RFID water filters: Some refrigerators use authentication chips to reject compatible aftermarket filters that lack the required RFID identification. This does not necessarily mean the manufacturer's filter provides better filtration; it can simply mean you are locked into buying that particular brand.

Self-cleaning ovens: The high-temperature pyrolytic cleaning cycle is a legitimate feature, but it can put considerable thermal stress on components, particularly door locks, thermal fuses, and control electronics. As an appliance repair technician, I can tell you that this feature helps keep people like me in business.

Air fry versus convection: Air frying is essentially a form of convection cooking. Both use a fan to circulate hot air around the food. Air fryers and air-fry modes can differ in fan speed, airflow design, and heating configuration, but the underlying principle is the same. Manufacturers have simply found another way to market a familiar technology.

Coffee machines built into refrigerator water dispensers: Another feature that adds complexity, cost, and additional components to an appliance that already has enough things to break.

Android tablets built into refrigerator doors: You do not need a tablet on your refrigerator. At all. You can enjoy the technology if you want, but integrating a general-purpose computer into an appliance creates another potential point of failure and another expensive component to replace when the refrigerator itself may still have years of useful life left.

Wi-Fi connectivity in washing machines and dishwashers: What exactly do you need it for? You have been washing clothes and dishes your entire life without an internet connection. You do not need a notification telling you that a cycle has finished. You can check the appliance yourself.

The appliance industry has become increasingly focused on adding features that look impressive on a showroom floor rather than improving the fundamental functions of the product. I would much rather have a well-built appliance with reliable components, straightforward controls, and inexpensive replacement parts than a refrigerator with a tablet, a washing machine connected to the internet, or an oven that requires a technician after running its self-cleaning cycle.

Build better appliances. Make them reliable, repairable, and affordable to maintain. Stop confusing more features with better engineering.

Source: I'm an appliance repair tech for 22 years.

Okay; lot's of memey answers here.

But I saw a video from "Technology Connections" and as I understand it most popcorn buttons are just timers.

However some microwaves have weight sensors and hydrometers. Most microwave buttons suck but some are actually good.

Another thing to watch out for a new microwave is whether or not low power mode means 20% power or 100% power 20% of the time.

I use the popcorn button anyway and never have any problems. I think the popcorn makers just want to spite the microwave makers.

shrinkflation. original popcorn bags =~ 3.5oz.
today : as low as 2.5oz up to 3.3oz.

the machine was programmed for a certain volume of food. it changed. most machine's have an adjustment for amount of food. there's only one variable on the machine we change - time.

I've never even seen a popcorn button. Is this a 'Murrica thing?

I've never encountered a microwave with a popcorn button, that seems oddly specific...

The Hyperfixed podcast covered this recently too

This post lacks [US] tag.

I use the popcorn button and popcorn button makes popcorn.

Better yet: Why are microwave interfaces so complicated? I don't want twenty buttons for different foods, or press up/down buttons fifty times because you didn't include a number pad

This might be controversial, but manual popped is better than microwave.

Microwave design in America infuriates me ever since moving over here.

Used to be you just press the number for how long you want, and then enter the power and 30% power meant max power for 3 seconds, then off for 7.

Then they stopped turning on the microwave when you press a number and you had to find "cook time" among the 15 different buttons for specific types of food that never turns out right or require you to look up a chart, it was faster to just press "add 30 seconds" 8 times.

Then they removed the "add 30 seconds" button.

Every evolution was more expensive and fancier, but replaced the most basic functions with things that I have literally never used or heard of somebody using.

Then I came to asia and half the microwaves have two dials: Time and power, and power actually lowered the heat instead of just boiling and then letting it wait for 10 seconds.

Liability.

https://m.youtube.com/watch?v=Limpr1L8Pss&t=47s

Long ago, Popcorn Button dishonoured himself and his family by breaking his word. Popcorn challenged him to battle, but Popcorn Button was a coward, and fled. Popcorn pursued him across valleys and over mountains, to the edge of the sea. There, Popcorn fought Popcorn Button for twelve days and twelve nights. No clear victor emerged, and the feud continues to this day.

if it burns down your house they can say they told you to not to use it and not be held liable

I use the popcorn button every time I make a bag. The sensor has never worked and I just have to pull the bag when the popping slows down, but the button sets a three minute timer and starts the microwave, it's easier than setting the timer and hitting start like I had to on my old microwave....the struggle is real /s

Acthually, the Popcrn button was intended for making popcorn Not in the Microwave bags. So when people use those bags and press the popcorn button it doesn't work out.

Because the original popcorn buttons weren’t very good. They mostly just ran normally and just used a questionable-determined time to run. This resulted in them usually burning the popcorn.

Modern popcorn buttons use a sensor to figure out when onto shut off, and they’re generally pretty good at not burning popcorn.

The warming remained, however because it takes money to change and they can’t know if you still use a dinosaur from like the 80’s anyways.