I actually thought of it more as a purely visual combine. So each post still lives in its own instance, and visually you just see them together. Comment threads would live on different instances and the instance mods just mods the community that they own. So it’s a purely Frontend thing.
I think “World News” and “Technology” are not quite similar communities. It’s up to the mods of each community to decide whether the content posted is appropriate to that community. One could argue, that an article about Threads is not exactly “World News” though. Also I think that the different variants of e.g. Technology will have a “flavor” of the instance that it’s hosted on. You then get the option to subscribe only to the flavors you like, or if you subscribe to all, then there’s bound to be some duplicates. Maybe some future feature could combine them - it would need to be clear which comment threads are from which instance though.
I have a few communities that I lurk in as well and I use a clean RSS feed app to subscribe to sub’s old.reddit RSS feed.
I use the app NetNewsWire on iOS which is free and clean. If you then want to follow technology subreddit you add the feed https://old.reddit.com/r/technology/.rss into the app.
Hope that helps some fellow lurkers out there.