@MIC97 Go to mods > update > update.rpf > x64 > levels > gta5. You should see this "popgroups.ymt". If you wan't add-on cars or peds to spawn automatically, you will have to edit this file. Extract the file to anywhere you want and edit it.
Use CTRL + F to search for the term "vehGroups" without the quotes. You will see the group "VEH_POOR".
For an example if you want to add an add-on car, do this;<Item><Name>YOUR VEHICLE CODE NAME</Name><Variations type="NULL"/></Item>
Put it in line in one of the groups in the file. Here are some of the groups;VEH_POOR VEH_MID VEH_RICH VEH_HAULAGE VEH_UTILITY VEH_FREEWAY VEH_LARGE_CITY VEH_COUNTRYSIDE_OFFROAD VEH_COUNTRYSIDE_ONROAD VEH_BIKES
Just know that each group serves different area of the map. I'm sure you wouldn't want a super car to spawn on the countryside off-road.
The same applies to add-on peds.