Gerrit uses a “Change-Id” trailer with a unique value. When you “fix up” a commit, the commit SHA changes but the change id remains the same. That’s how it can identify different commits with the same change id as patchsets of the same change.

This is based on what I remember (haven’t used gerrit in a while), so it may not be accurate.

I used gerrit in my previous job and miss using it. Would definitely prefer it over GitHub which is more popular (and convenient of course, can’t deny that).