All code needs to be reviewed and approved. The official technical rule is something along the lines of anyone can approve anyone else's change (to anything) so long as they both aren't interns.

Socially, it ends up being more like "someone who maintains and cares about this code should approve it" almost all of the time.

Not sure how it is on the prod side, but certain chunks of infra may also have designated "owners" and you needed to have someone on that list approve the diff to land it (e.g. rando engineer can't get his buddy to approve a diff that changes the push process...unless that buddy is chuckr or someone more heavily armed than chuckr)