zipmod file you need just to put the file in "mods\MyMods" folder. zipmod files you need to have a properly modded game such as BetterRepack, HF Patch, or your own self managed modded game with sideloader. Zipmods are an easy way to add assets like clothes, items and maps to the game without generating ID conflicts and messing around with the game structure. Some good hearted developers put the right folder inside their packages, so if a plugin file have BepInEx folder you just need to extract to the game main folder.
dll extension and must be placed in "BepInEx\Plugins" folder. They are mainly used to add clothes, items and maps to the game. zipmod extension and must be copied to "mods\MyMods" folder.
Note that when you update the Sideloader Modpack all mods that are different from the community list are excluded, so don’t put your own mods in Sideloader Modpack folders. In an modded Honey Select 2 there are several "Sideloader Modpack" subfolders in "mods" folder and are managed with the update tool. This package is made and maintained by the community and contains mods that are open to distribute.
This approach is likely to introduce errors and if any mod breaks your game it’s hard to seek the problem and its more likely that you'll need to reinstall everything again.