ive seen quite a few car mods in a cruise server "just a ride" (and more servers like this) that are not even on the mods list. i did capture some of their names:

-vrc fa 2007
-mersoles ce66s
-poscher 819
-both iconic bmw m3 gtr and ford mustang from need for speed mw which i didnt get their mod name and more...

so my question is where do these mods come from? do users make them for themselves?

There is a 'custom access' setting, ie, the mod only shows for people whose usernames have been added to a list by the mod creator.

(Some people apparently get all the way into adulthood without ever having to learn about sharing, bless 'em)

Quote from Yanny :

ive seen quite a few car mods ...

The top three are all in the list and downloadable - I would check you don't have a filter on.

POSCHER 819 is marked as WIP - make sure you enable "include WIP mods" in filters
MERSOLES CE66S is not approved - make sure you show "all" instead of "approved"