Vast majority of streamers are in fact servers as well. There are also streamers that don't have the capability of acting as a server, these could also be called renderers. A streamer with server capabilities has the capability of storing your music library and running music player apps such as Roon, Audirvana, etc, some run proprietary apps as well., this interfaces directly to dac or 'converter' as you called it. Then we have streamers/renderers incapable of storing libraries, these devices act as renderers/endpoints for music player. Essentially, all streamers are computers modified and optimized for a single purpose of acting as a music player via music player apps. The streamer/renderer is simply a more elemental computer than the streamer/server.
The streamer/renderers were designed for a single purpose and that is to act as an output for various music player apps such as Roon, HQPLayer, Spotify, Squeezlite, the idea being by segregating the output from general service computers or other streamers sound quality can be improved. An example of this, and what I run is, Roon Core on custom build streamer/server, Roon Endpoint on Sonore OpticalRendu, the Euphony operating system running on my custom build calls this Bridged Mode. I can also run both Roon Core and Endpoint on my custom build streamer which negates need for the Sonore OM. In my comparisons running Roon on two streamers provides superior sound quality vs running on one streamer.
So the above a relatively complex setup, three discrete components optimized for a single task. You can also go the all in one box route, streamer/server/dac integrated for direct connection to dac. And then there is streamer to dac setup, this likely the most used setup.
Seems to me you have a pretty good understanding of how things work, its the discrete renderer/Endpoint component that confuses people.