how do you search lemmy comments ?

I guess you can't. The app is still in alpha, after all

You can't atleast not yet . There is a PR by MV-GH open but its still in works last time I checked .

• on the Communities page https://lemmy.ml/communities the Search bar just searches community names
• on any page though, the Search icon (magnifying glass) in the top right corner takes you to the search page https://lemmy.ml/search
  • the “All” menu lets you choose to search just posts or just comments
  • the “Local” and “All” buttons restrict your search to your instance or to all of Lemmy (that your instance is federated with)
  • “Community” and “Creator” menus let you restrict to a specific community or creator