Hello, mods are compatible for 1 version only, sometimes you get 1.9.4 & 1.10.2 support but it isn't common. 1.12.2 and 1.13+ are very different as Forge got restructured so... best to get used to being particular with mods unless the developer states it supports many like the 1.9.4/1.10.2 situation I stated before or Fabric working across all 1.14 versions as another example.
Also be careful downloading mods as mod repost sites exist and they can be dangerous. So best to stick with Curseforge as there is no ad site redirects, malware and otherwise in the files as every file is checked by people at Curse.
If you want to edit a modpack just go to the mods folder/mod lists and remove (or put .disabled after the .jar when renaming the file).
Modpacks work in modpack launchers like Twitch, Technic, FTB, ATLauncher, MultiMC etc. So if you are installing one expect to use a modpack through the Vanilla Launcher and dropping the zip file in the mods folder after you have installed Forge it won't work. Modpacks are a collection of mods and are package to be then unpacked through any of the launchers I stated above. You can just put mods in a zip file and past them on to someone but it's better to use a launcher as any modpacks downloaded from modpack launcher sites or curseforge with have particular files in them and expect a launcher to unpack and install it for you to play.
2.To install and older Forge version just get it from the Forge website, to select an older version you have installed already just go to the instance/modpack (MultiMC, Twitch)/installations (Vanilla) tabs and change it by editing the modloader.
3.Forge should work on any OS as far as I know. The jar file method is recommended and more suitable for Mac/Linux, if you have Twitch or MultiMC I'd recommend using those as it makes Forge or other modloader installs much easier and better for managing.
I'd suggest watching a Forge install tutorial or a modpack/launcher video to get an idea.
If you'd like images to better explain things let me know and I'll present some.