How do I do that from the web browser version of lemmy.world?
I try going to that site and can't subscribe unless I'm a member of masto.ai. If I search for user on lemmy I find him, but there's no obvious subscribe mechanism.
I'm no expert here, but that looks like a Mastodon link, and Lemmy itself doesn't federate with Mastodon. They both use activity pub, but it's not the same content. I think kbin does both.
Relations with other online social networks
Lemmy instances are federated with each other, allowing a user on a given instance to participate in a "community" on another Lemmy instance without having to create an account on the other instance;[4] and with other instances in the Fediverse that use different software. ActivityPub is the protocol used to allow Lemmy instances to operate as a federated social network. It allows users to interact with compatible platforms including Mastodon and PeerTube.[14]
I mean when I search for user, his page shows up, just don't see how to subscribe...
Again, I'm not an expert, but I believe the issue is that, while they both use activity pub so they can theoretically interact, not much of that functionally has been implemented yet. I'm reading that mastodon users can follow Lemmy communities, but I'm not seeing anything the other direction.
Probably a good feature request, though right now I think the developers are busy fixing the bugs uncovered by this month's unscheduled stress test.
You won’t, the format doesn’t really work well together. You can technically do some interactions between them, but there is no client that makes it easy for you.
Don’t stress about it. Just think of them as separate.
Kbin doesn’t have an API yet, so most app developers don’t go for it, the only one I know of is Artemis, but that one doesn’t support Lemmy yet (it is planned).