How does sunscreen actually stay effective?

First of all, I wear sunscreen religiously on every bit of exposed skin every day of the year.

That being said, there are some things I don’t understand.

If sunscreens start to break down in the bottle under high heat (not that high- 86F or 30C is usually what it says on the back), then how does it not quickly break down and become useless on your skin when you’re out in the sun in the summer?

Also, I have heard that mixing anything with sunscreen can disrupt the film, rendering it ineffective. Then I hear dermatologists suggest that if you want to put on anything else, like a serum or moisturizer, you can put it on, let it dry, and then apply sunscreen. But hello, that’s obviously going to mix with the sunscreen. If you put anything wet on your face after putting a product on, even hours later, you can already feel it mushing around your face again. So is that sunscreen now also useless? (Incidentally the only thing I use on my face is sunscreen anyway. My body I moisturize though.)

I just don’t get, if sunscreen is so sensitive to factors like these, how is it not rendered ineffective in a few minutes or instantly on most people?