Almost all of their changes are UI changes. They use our core OBS Studio code, with an Electron GUI instead of Qt. Not easy/basically impossible to port back over. We monitor any back end changes they make, and pull when appropriate. They do not collaborate with us, however, so it's rare they make changes that we can use.

I think one of the reasons why is because Streamlabs sells premium add-ons https://streamlabs.com/goprime

That aside, I think it specializes OBS in a way that is too specific for OBS, which tries to be more generic. I think they both have their place, but personally I use OBS for recording videos and SLOBS adds no value for me.

I didn't know about slobs, but the idea of a "theme store" maybe would be something worth to explore on obs, It actually could be a revenue stream, just like wordpress, where the core is open source but you have several theme stores.

It's a frontend for OBS.

This is the Electron app: https://github.com/stream-labs/streamlabs-obs/

This is the wrapper library for Node: https://github.com/stream-labs/obs-studio-node