How to prevent burn-in on iMac Screen

Somehow my 2015 5K iMac screen is getting burn in, and I swear it happens sometimes when the screen isn't even on. Oh, and also, the burn-in can take affect even if I'm on my Mac for just minutes; it doesn't even have to be a long period of time. I think it's temporary, but I'll find out for sure. I think the main reason for the burn in is when I'm doing web-development and can't be in full screen. Anyway, does this happen with all iMacs or is this just something with the older iMacs, or is just because of my iMac's age? I'm just curious so I know what I should/shouldn't do if I end up getting the new M3 iMac later this year or early next year.

[Re-Titled by Moderator]