You can append (intext:“modlog” & “instances” & “docs” & “code” & “join lemmy”) to your search query to search through all Lemmy instances. Works with Google, Startpage, SearX.
People are already working on unified search tool, it’s not really hard as all you need is a list of instances and and a way to index them.
On apps search functionality will vary based on app, but I would expect most to add the the fuctionality eventually.