@glycbz We have a bunch of modding tutorials around here. Just look at the stickies here and in "General modding". Some people have put a lot of effort into writing them. Don't disappoint them by ignoring those.
About how the game knows about new cars and what to load:

There is a file named extratitleupdatedata.meta in update.rpf/common/data. All patches are registered there. There is a priority system. The game loads all registered DLC patches and gives them top priority over any other data. The mods folder is maintained by OpenIV. It hooks into the game (you could say it detours the code) and tells it "hey the mods folder has priority now" (simply put). The OpenIV.asi is responsible for this.

I have made a detailed tutorial about how to create your own DLC patch.
https://forums.gta5-mods.com/topic/14753/tutorial-how-to-modify-dlc-data-without-any-hd-space-overhead-your-own-dlc-patch