No reply to the referee report from the author of a manuscript
It depends on the journal. The journals I've been associated with have an online system that gives authors a certain deadline by which they have to submit a revised manuscript -- I suspect that the deadline is often set at 6 months. Authors are reminded of this deadline by an automated email a certain time before the deadline expires. Authors can ask for an extension, and it is routinely granted.

If an author does neither, then the manuscript is moved into an "archived" state where the database still has it, but it is not considered a "live" or "active" manuscript. Unless someone takes action, that is the final resting place of these manuscripts.