Indoor or outdoor paint for wheelchair ramp inside garage?
It's pretty much a porch, despite being in the garage, so I'd use porch paint on it, at least for the floor/stair treads - with grit or traction aid of some sort. It will get water from foot traffic when it's wet out, to some extent.
I would not choose stain in part because you have a large expanse of plywood, and stained plywood is not a look I'm fond of. YMMV - and in this case the first M is Mother, not Milage.
If the paint wears, just as with a porch, easy to add another coat.