As with all mammals, mother will feed her offspring.
They will start to ween off her at about 6-8 weeks, though you might find that they continue occasional suckling for a while after this. Until then you should only feed the mother (high protein diet is good, but any suitable food will do). Once they start to ween, kitten food will be a solid bet. Dry or wet, whichever you want to feed them. Kittens are adaptable creatures.
If it were your pet cat, and she felt that your scent was familiar then you'd be fine to handle the kittens. As it is, probably best not to touch them for a few weeks. If and when the mother is comfortable with you stroking her, you're probably fine to interact with the little ones.
You're right that the cat feels comfortable and safe in your garage. Of all the places she had been she chose to have her kittens in there. Shelter, food, and a creature (you) that she doesn't fear. For now, best that you leave nature to take its course. Once the kittens are about 10 weeks old you can look to rehome them (unless you want to keep some of course!). I hope you will think about keeping the mother cat though... she can always be an 'outdoor cat', but it's clear that she trusts you. Cats adopt their owners as much as the other way around!