theres...alot... to say the least lol. biggest thing is to make sure you are editing inside your mod folder only, we all mess up installations every now and then and sometimes the mod folder just breaks and you have to delete it and remake it. being in the mod folder ensures that the base game files wont be affected. im going to attach some screenshots of what my mod folder looks like so you can see what files should be put in there for everything from cars to sounds. x64e is only there because of the graphics mod I use. chances are you probably dont have x64 in the mod folder so go into the main game directory and copy the x64 file and paste it into the mods folder. this file is where the sirens i use go.
When it comes to vehicles, there are two ways to go. add-on or replace. add-on vehicles will require a trainer to spawn (i use enhanced native trainer) *be aware that most trainers will share a keybind with the RPH console so once downloaded, go into the trainers .ini file or config.xml depending on which trainer you choose and switch the keybind from F4 to something else if it conflicts* i changed the main menu toggle from F4 to F5
add on vehicles will also require 3 additional items to not crash your game. these are
-game config for limitless vehicles
-packfile limit adjuster
-heap adjuster
all 3 can be found on gta5-mods.com
vehicle replacers will replace the default police vehicles with what you install. it really comes down to personal preference but be careful because sometimes when you replace the vehicle.meta, something gets corrupt or wasnt edited correctly and when a vehicle spawns it crashes your game. i had an instance where a replacer had something wrong in the meta file for POLICEB and anytime a police motorcycle spawned it crashed my game so i ended up deleting my mod folder and starting over. its relatively rare for something like that to happen so dont worry too much about it.
most mod makers will typically give you a readme with the pathways needed for proper installation so it makes it rather easy. Sirens, Vehicles, EUP, Graphics, weapons, etc. will go into your mod folder with open IV. plugins and scripts are typically drag and drop. typically you will just drop the included plugin folder into the main directory or into plugins/lspdfr, but ALWAYS read the read me just to be sure. it can be very confusing at first but it gets really easy once you get the hang of it. if you need more assistance or recommendations message or respond to this and i can offer more help