Some clients allow you to pin the public timeline of another instance. As for spreading beyond your home server, tags are key. With the right client, or the Mastodon web UI in advanced mode you can pin a series of tags as its own timeline, which updates in real time, so you can follow discussions of interests, grouped together, with results from across the Fediverse.
That sounds very nice and I must admit I haven't made a lot of research on this topic. But today the major servers are still sold as a building block for an "inside" community. You browse instance that are categorized by interests.
Basically, the domain has much more significance than it really needs
