And you get the data through API calls - so in theory one can build your own UI on top of a known service. There are rare examples of this done succesfully (for HN for example), but usually you won't get very far in a reasonable amount of time, because often it is a mess behind the shiny UI. (And because this is not encouraged behavior by the service provider)
another one that i'd like to think has an open enough API is mastodon/fediverse, assuming they actually adhere to the activitypub spec, it should (should) be relatively straightforward to write a UI on top of the API. And it seems ok, there's quite a number of mastodon "apps", but less for stuff like misskey or pleroma.
I'm sure there are other contemporaneous examples, but suffice to say that some developers still care about this, today!