How do I fix the stairs to the garage where the treads look like they are about to fall?
When you pull apart the two sides of your staircase, either side can come loose. It looks like the left side is tight against the wall, so I'd hammer the right side back in (use a piece of scrap wood so you don't leave a bunch of hammer marks).

I'd then find a way to secure the bottom tread with screws into both sides or secure the right side to the floor by running a tapcon screw through it and into the concrete.