How does owning aircraft / military vehicles work in GTA Online

The "S" mean that those cars need to be stolen and brought back to Simeon to be available. And yes you'll need a garage to buy cars online.

Installing a tracker allows you to see your car on the map. You can install a tracker at LS Customs, or you can get a free one when you store the car in your garage.

A tracked car can still be totaled or stolen. If this happens you are out of luck. Buying full coverage on your car at LS Customs gets around this. If the car is lost or stolen you can call up the insurance company and file a claim to get it back.

Each garage can hold a set amount of cars(between 2 and 10). Most cars you can steal can be stored in the garage. The game is a little restrictive on the high end cars-you have to buy them. I haven't tested it, but I assume you probably won't be able to store the bigger vehicles like semis and cargo trucks either. Physically they are probably too big to fit in the internal volume of the garages(at least the 2-car one I currently own)

Another bonus of the garage is you get a mechanic who can deliver a car from your garage to you anywhere on the map for $50.