Why is there a channel cut into the bottom of this door? How to seal the gap?

Question

I have a fairly large gap between my door and the threshold (picture 1). Additionally, there's a gap between the door and the side frame (picture 2). Also there's this large channel in the bottom of the door (picture 3).

What would be the best way to seal this up for better insulation in winter/summer and to prevent pests from entering? This door opens to an interior hallway shared by other apartment units. What is that channel in the door for and what is it called? It has holes for screws so I am wondering if it's meant to install something there that will seal the gap at the bottom.

This is a rental so I prefer to not drill anything. I looked at adhesive door sweeps but they would not seal well since the door actually sticks out slightly from the threshold (picture 4), assuming I only apply to the interior side. Perhaps I could put two sweeps, one on each side? I think an under door slide on sweep could work well but most are not tall enough (and it won't seal the side). There's also these pool noodle ones that seem like they would seal well since they cover both sides.

Answer 1

This door looks like it could have been set up for an automatic door bottom which would push down to block the gap.

Also, there are other forms of door sweep that could work better than your linked items. McMaster Carr often has unusual items that could be searched for from cheaper locations. McMaster Carr door sweeps (Not an official endorsement, but I have used McMaster Carr a lot and it's been a good source for me. So maybe I am endorsing it...)

The side gaps could be handled with stick-on weather stripping against the door stops.