Most free modules are open source. (MIT is the most common license).

Premium modules are closed. You would just have to check each one. (Hundreds upon hundreds of them).

It is a very good helpful community, I can find and talk to the developers so fast.. bug reporting and fixing, requests for just about anything needed if it is helpful I can get upgraded so fast. Literally hundreds of developers all working constantly to make the vtt experience amazing.

Their discords and githubs are so busy and fun. Foundry is very addicting. I have used Roll20 and its pretty good, but Foundry is much deeper and my favorite. I have seen a little bit of Fantasy Grounds Unity and it looks amazing as well.

I just have so much invested in materials on dndbeyond.. then add-ons for roll20, and now Foundry. Premium modules, premium quality animated maps, tokens, quality of life add ons, so much more. Fun times.