how exactly [...]

Sure, UUIDs are a useful tool. What of it? If I put a UUID in a comment, it isn't a link. This doesn't answer my question or solve the problem. The link has to go somewhere on the web, or use a custom protocol specifier and be handled by a client application or something installed on the user's machine. If you go the client app route, many/maybe even most people use lemmy in a browser at least some of the time, and this will never get the full adoption required to make it standard. If you go the web link route, then you have concerns like "who owns the domain/service that does the redirecting" (ie matrix.to), can they be trusted, how can they automatically tell which instance to send users to without privacy concerns?

If you're proposing overhauling the whole architecture of lemmy to use consistent UUID-based IDs for comments, posts, etc. across all instances, that could probably work but there are some edge cases especially with malicious actors, and it would be a huge undertaking.

A better idea, IMO, is to let client apps/frontends handle the translation, so that regardless of what instance the comment is linked on, it is translated to the correct local link for local users (unless the instances aren't federated), since there's already the fedilink button to then see the post on the original user's instance, but there are probably edge cases and performance issues I'm not thinking of/privy to, and its still a non-trivial fix, which is why it hasn't happened yet. I'm sure the devs would welcome such a change if a PR were submitted with the kinks worked out, but it isn't on their current priorities list afaik