How can I coat a garage apron slab to make it smoother?
There is a product called "concrete resurfacer" which I would vaguely equate to "the concrete version of asphalt driveway sealer" - don't use asphalt driveway sealer on concrete...

You first patch any actual damage, then mix up the resurfacer into a slurry of specified consistency, and squeegee it onto the dampened slab to fill in the minor imperfections. You can give it a finish from troweled smooth to brushed non-slip.

These products should not be used to fill expansion joints - those can be filled with flexible rubber caulking compounds made for the job, if filling them is required.