But how do you find a website and the relevant content then?

Not sure that I have a good answer there but, with the rapid decline of major search engines because of SEO, paid result placement, and LLM-generated ad sites, I'm going to go with "probably not a major, ad-funded search engine". At the current rate, it is likely that it wouldn't even appear in the first page or three, even if clearly relevant. A non-commercial, community effort just can't compete like that.

SEO could also just mean that all the stuff is actually server side rendered, so that google is able to find the content (which is not the case yet for lemmy-ui, but there is lemmy-ui-leptos in development, which should support that at someday)

SEO, in my experience having worked in the web hosting industry, has pretty specific meaning - gaming the search engine algorithms by using keywords, networks of unique IPs pointing to it, etc, almost be always with the intent of getting ad pages higher in the search results.

Yes, not being able to be rendered outside of ActivityPub easily makes it unlikely to appear in Google. But, that's not necessarily a bad thing and can be a forcing mechanism to change how we are interacting with the Internet and reduce the amount of participation in providing profit to those selling our personal information. Plus, there are many of us that don't want to contribute to learning data used to power LLMs that being used to worsen human life for profit.