There are plenty of studios that make lots of assets for other studios. It's called outsourcing and it's usually handled by studios in countries with really low wages so they can be done for cheap. Naughty Dog outsource to several Mexico for example (mostly rough props and levels that get polished by the internal devs) A Veitnamise studio called Glassegg does most of the cars seen in games lol including most of the Forza cars and several tracks and all the recent Need for Speed cars etc. Most of the major publishers have studios in places like China to do a lot of the grunt asset work, EA, Ubisoft, Sega and 2K games all have studios in China.
Alternatively for smaller stuff their is the Unity asset store and several other places where you can put up assets that devs can buy.
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