It’s a set of components (projects “hub.sr.ht”, wiki “man.sr.ht”, git repos “git.sr.ht”, mercurial repos “hg.sr.ht”, build service “builds.sr.ht”, bug trackers “todo.sr.ht”, mailing lists “lists.sr.ht”, I think that’s the main ones at least) which can be installed separately as needed (though at least the wiki is powered by version control so I think it needs at least one of the repo services). Everything created in these components is also separate except for projects which tie together multiple repos, lists, issue trackers and so on (for example, see sr.ht/~sircmpwn/sourcehut/ which is the main sourcehut project), as opposed to a strict “1 issue tracker per git repo” as with gitlab and a lot of the other “all in one” services.
It also emphasizes the git send-email workflow as opposed to the (in my opinion, supremely convoluted and annoying) PR workflow, which means you can just clone the repo, make your changes locally, and send patches. (Though you can still fork, push and then submit patches from the web interface if you want to, to get something similar to PRs.)
So KDE at the minimum could just have hub + git + lists (for patches), and keep Bugzilla as the single bug tracker, for example.
The builds/CI service is also supposedly excellent, but I have to say I haven’t used it yet and KDE is already using Jenkins anyway. Though of course, it does integrate with the other parts, CI for each commit and incoming patch set which is still useful to have. I think KDE is currently using GitLab CI/CD also.
I think that’s the main big parts, there’s more info about its features on the main website: sourcehut.org.