I know. Wefwef doesn't belong to instance tho. The reason I asked this was, I wanted to know if the instances are restricted to use the default UI that lemmy provides
With enough nginx config you can probably get around it. You can send certain requests to certain programs and other requests to other programs.
So even if you had a server, with a particular domain/IP, and if lemmy was built to only run the API server alongside a web server whose client consumes from that API, you could reroute any requests that would have gone to the web server, to a wefwef instance instead.
Sorry if the tech jargon’s beyond you.
As an analogy, even if a McDonalds instance is a package deal combining the kitchen and the eating space, there are ways you could make it so that anyone showing up at that street address sees a Starbucks that can get you a McDonalds burger.