Welcome! GTA modding isn't as nicely integrated as TES/Skyrim. As far as I know, there haven't been any real mod collections. Mod creators often do link related and/or recommended mods to use, but there's no proper (existing) collections.
Vehicle packs do exist, such as VanillaWorks Enhanced or IVPack, but are scarce, mainly due to most pack attempts either do not contact the original creators and/or cobbling together performance-unfriendly vehicles.
A mod pack, called Redux exists, which combines lots of components, but is shunned by the community because initially it used stolen assets, re-labeled as their own work. I am not up to date on its current status regarding that.
Could be a nice idea to start this effort yourself!
Regarding the mods folder - yes. Say you want to replace a file in x64e.rpf, you need to copy that into the mods folder before working on it. This applies to any file: first copy the rpf file into the mods folder, and then only edit the file in the mods folder.
When you install an
.oiv file with OpenIV, it will automatically copy the needed rpf files, provided you have a mods folder present.