The eli15 answer is that concrete is repaired by excising the damage and replacing what was removed. If you're really interested check out Sikas brochures.0 if you prefer video form I found this [1] that seems to cover a good variety of applications and can probably be comfortably watched at 2x speed.

Maintaining concrete is a matter of having routine structural inspectors come out to poke and prod the concrete. Worn traffic areas get patches and sealants. Areas with a lot of runoff get seals to direct water in ideal ways. Etc.

0. https://www.sika.com/en/construction/structural-strengthenin...

1. https://youtu.be/c0Pj5Qdy9IU

Probably the most important one: keeping the roof from leaking. Uncontrolled moisture incursion is one of the quickest ways to destroy any structure.

plugging/checking for holes in the roof so there's no water infiltration in the walls (water in the wall will quickly degrade them, especially if temperatures to freezing levels)

Pretty much. Water is the enemy of any construction.

A farm near where I grew up had a big barn, built in the 1700s that was in daily use until the mid 80s. Due to a family/inheritance dispute, maintenance ceased in the 90s. At some point, the roof leaked and that set in motion things that resulted in collapse last year.