Basically, the mineral components in the soil - whatever rock the soil particles came from - undergo chemical and physical degradation, releasing micro nutrients bound within them.

Potassium is a good example even though it's a macro nutrient - the mechanism is the same for the most part

Rock is the slowest but largest pool of nutrients like K. With K in particular, clays can bind it and desorb it depending on clay type and hydration status. That K is generally more available to the plants.

Think of it this way: you have a mountain with diamonds in it, and a pick axe. That's your mineral K. You can chip away and get more diamonds but it's hard to do so and time consuming.

There's also a smaller pile of diamonds that you can just sell to people. That's your available K.

Then there is diamonds you are getting polished, that can later be sold. That's your clays

https://nutrien-ekonomics.com/news/how-can-the-nature-of-clays-in-your-soil-affect-plant-available-potassium/