Why does a big yellow spot appear on my screen, even when it's off?

Would be very helpful if you could provide some photos. With the screen off as well as on and how it looks over different things.

Permanent burn-in can affect a screen if you had a yellow spot on the screen and the brightness up to the highest, you could have permanently caused a "burn-in" where that stark part of the image is partially or fully forced into the pixels. Alternatively, a physical burn can cause spots on an LCD. Such as a lighter flame or an overheated chip on the logic board.

EDIT: Thank you for providing a photo.

That definitely looks like a physical burn. That section of the screen has nothing really behind it that should generate excess heat. The back of the screen is just a plate covering data cables, and inside the phone case is where the microphone is located and really not much else under that spot.

My money is on external burn such as you set it on the counter and it accidentally touched a corner of the hot stovetop, or got it too close to a lighter or something.

The good news is this kind of burn is basically cosmetic only, so as long as it's not big enough or obtrusive enough to prevent you from using your phone, you're just fine to use it as is. The bad news is if you want to fix it, it will require a screen replacement, which is quite expensive on the high-def screen of an iPhone 13. (For reference, $375 in my shop for OEM grade OLED screen, or $210 for an In-Cell LCD. I would wager prices at a shop local to you are roughly the same if using equal quality parts.)