Generally, personal items don't raise any eyebrows. I've carried my own gadgets widgets and doodads through customs in a bunch of countries, and nobody ask any questions.

I also b(r)ought a laptop to Brazil for a coworker there, so he could avoid tariffs, and that was fine. But I made sure to get rid of the original packaging just so it wasn't obvious that I was bringing something brand new and recently purchased abroad.

You will likely have to pay tariffs if you declare more than $800 worth of goods at customs, as you are required to by law. You declare them based on what you spent to purchase them, not what equivalent goods would cost in the US. Of course, you might also be the kind of person who bought their ps5 in the United States and then took it with them on vacation to do some light gaming in the hotel and is just bringing it back with them at the end of their trip; I couldn’t possibly say.

Generally speaking, you are allowed to bring a small amount of items you buy abroad back to the US without paying duties, as long as you declare them. Looks like that amount is still $800, in spite of this dumb Administration. This doesn't apply to everything, though, you already know about alcohol and tobacco but there may be other things that don't count against that $800, and it's your responsibility to know. (And some things are banned regardless, like fruit and some meat products).

You arw allowed to bring personal items you bought in the US back to rhe US. Back 30+ years ago coming across the border with electronics would be suspicious, but now people regularly travel with laptops, phones, portable consoles, and all sorts of other expensive electronics. Having another single item, not in it's retail packaging, would hardly bat an eye.

But, I will say that disclaimer that you "have to declare the item" is a bit iffy now. It used to be that the US forced everyone to submit paperwork with whatever they brought back with them from abroad with every entry. But, I think the Pandemic ended that. Now, it is very easy to simply leave without declaring anything. I would say that if you wanted to bring the thing back in it's retail packaging, it's worth taking the time to declare it, and just make sure you have a list of all the items you are bringing back. The customs agent may just wave you through once they are satisfied you are not reselling anything, but at least you know it is legit.

Depends on the country. They all have their rules published on websites which you can check.

Feel free to not declare international purchase at your own peril. CBP doesn’t search every bag. Sometimes if your declaration is close enough to the limit they don’t care. However if they do search and you haven’t declared, you’re going to have a longer border crossing than normal and very likely that will be the case in the future.

There are tariffs on many or most things under this administration. No matter the form of import (mail, carried personally, courier), you will need to pay the tariff. Purchasing it internationally (eg japan) and importing to the USA triggers tariffs. Sony pays the tariffs at import and passes the costs on to the consumer if you buy a PS locally