The Meaning of Life, the Universe, and Everything.
If I have mods in my mod folder from 1.12. And I decide I want to play some mods on 1.15.2 so I add 1.15.2 mods to the mod folder. If I play on 1.15.2 with the 1.15.2 mods will only those mods appear on the forge mod list or will it have issues with 1.12 mods I also have? I'm new to mods and still trying to figure out how they work.
Always make separate folders for each version of MC you want to play.
Suggestion: use a third-party launcher like Twitch or MultiMC to play modded MC. These apps make it very easy to create Profiles, install Forge and mods. The apps keep everything in separate folders to avoid conflicts.