There are several ways to prevent of limit the flow of air under a door. the most basic is a “Door Sweep”. It is a flap of rubber that fastens to the door and covers the opening when the door is closed. This keeps the draft out.
If you want a temporary solution for the colder months, this is a draft stopper:
All you do is push it against the base of the door, and the insulating material stops the cold from coming in.
But you don’t have to buy one, you can make it yourself!
Tip: many of these use pool noodles or pipe insulation for the insulating core.
Hoping you keep toasty warm,
Alan