Who is responsible for buying new doors at a condominium?

Read your governing documents to determine ownership of the doors in question.

Doors inside a unit are usually the responsibility of the unit owner.

Entry doors to units are generally considered limited common property (areas), and are replaced by the association. This affords uniformity in the architectural guidelines which are also available in your governing documents.

When an entry door is damaged and must be replaced, the association replaces the door. If the door is limited common entry property, and the association decides that the unit owner should pay for the replacement, then the unit owner pays.

Garage doors that afford individual unit owners access to limited common areas called (attached) garages, are usually replaced by the association and charged to the unit owner, based on the unit owner's solo use of the garage door.