I'm going to guess they cut around the damaged area and weld a new plate into position.
They might weld backing plates across the join between the new plate and the existing hull, then fill the gap with more welding.

Similar to how you fix a hole in plasterboard. Just bigger, heavier, and weldier.

Certainly not an easy thing to do. There will be bulkheads in the inside, wiring and plumbing, all sorts of things.