Steam

Its steam workshop, so if you are not familiar, all you need to do is own TTS on steam, download it, then go to the steam workshop page for TTS and then yeah its basically like you said from there. Any single thing you can find on the steam workshop can be downloaded for free and loaded into TTS. I have played hundreds (not even exaggerating) of games on TTS and I have never had an issue finding a tabletop game I was interested in on there.

Yeah basically. Some board game companies have made official DLC versions of their games (like Wingspan's is pretty good) but the community has added many more games in the form of mods you "subscribe" to. It's a great way to try new games before committing to buying them, and some of the mods even dramatically improve on the game (Terraforming Mars for instance has an amazing mod for it). Thanks to TTS I was able to add some new games to my collection with confidence and splurge on special editions because I knew I liked them and that they'd get played.