Usually adjust some dampers. In extreme cases they may need to resize a duct or add dampers to existing ducts. Basically they’re measuring how much air is getting sucked into your HVAC unit and how much air is getting pushed out of each register and try to make those numbers the same.

Start with replacing the weather stripping - there are multiple sizes and they are cheap so buy a couple and figure out what size fills that gap without affecting the function of the door.

If problems still persist, the other thing you can do is get an HVAC contractor to come out and check the air balance in your house. If you have negative pressure in your house, you’ll be constantly drawing air from the outside into your home.