There is also https://www.search-lemmy.com/ which seems to work pretty well. It's also open source
The future is here: https://www.search-lemmy.com/
As a matter of fact, I remember seeing this project posted about a bit ago: https://github.com/marsara9/lemmy-search
It can be selfhosted with docker or you can use the hosted version at: https://www.search-lemmy.com/
An announcement post is here: https://lemmy.world/post/963301
You don't need to make an account for other instances.
It's like email, just becuase you may have an email account with Gmail doesn't mean you are prevented from emailing Yahoo accounts.
If you have an account on lemmy.world, you can still read/comment/etc posts on other instances such as lemmy.ml. You may need to change the toggle from "local" to "all", but you should be able to interact with other communities
From a slightly more technical perspective - ActivityPub is the protocol(like how email uses MIME and whatever the RFC is lol). Any activitypub service can hypothically communicate with any other activitypub service.(In practice is another story) This includes lemmy, mastodon, peertube and even facebook's upcoming Thread service.