Similar to Mastodon's spikes last year, it seems. Anyways, there is data to think about. Source
Lemmy pretty much just needs tags. Like you can mark your different "technology" communities with the tag "technology" and a user can subscribe to this tag to view all posts from whatever communities have this tag (and they don't have to call themselves strictly just "technology")
Something like that I would imagine so no direct interaction between communities required.