Mostly everything that's addon dont apear in the game, that's why when you install an addon car you won't see it on the streets, you will have to spawn it using a trainer, but if you really want to see add-on cars on the streets there is a script mod that allows for that, the name of it is DLC/Add-On Vehicles Spawn on Traffic. As for peds weapons and clothing, you might need to use trainer for that, i don't think you will be able to see them on the streets unless you replace them, wich is a little work to be done.

So I am a RAGEMP player, there is this server that offers REDUX that works with ragemp, or it used to work before Rockstar Launcher, because it checks the files and replaces them. I order to use the mods with RAGEMP you need to overwrite files. So is there anyway to make the launcher skip the check for files? or anything like that?