How does the garage system work in GTA V?

Solution 1:

If you park a car in the garage, it will be saved there. Your standard car will always be available nearby if it is not in the garage. Parking in the garage will save that car and you can access there unless you left it somewhere else.

Taken from the manual:

Player characters can own any vehicle in [GTA V] as long they have appropriate storage for it. To purchase a new car they must first own a garage ... If a vehicle is destroyed it will reappear at any residential property that the character owns with a garage (note that this does not include safe houses). If an owned vehicle is lost, stolen or abandoned, it will be impounded. To retrieve impounded vehicles the character must pay a find of $250 and collect the vehicle

Solution 2:

All cars can be stored in garages. The standard car will always reappear near the safehouse.

I had times where my stored car disappeared and got replaced by my standard car (both Michael and Franklin). Of course, there is a possibility this was because a mission replaced said car early in the game, or I genuinely forgot the car somewhere else (hopefully the latter of the two).

The garages not tied to the safehouses work the same way, but you are able to access a special car (I have Special Edition, but this is likely for every version of the game).

Air fields should work the same way, unless they work like helipads (can be tested with another plane). Helipads MIGHT be able to store more than one helicopter. I am not sure of this.

After the IAA abduction mission, I flew back to Trevor's airfield/helipad storage area with the Frogger helicopter from said mission. I landed nearby, since the spot was already taken by another Frogger, however, before this mission, the helipad was empty: this was my first time flying a helicopter.

Going near the helipad, the game gave me the option to "switch helicopters" with the right DPad, however, the only choice available was the Frogger. This got rid of my other Frogger, right before my eyes.

I flew around with the spawned Frogger and came back, to see a completely different Frogger spawn. So I think that option was to set which one spawned. No other garage so far has given me that option, and I do not know how to add more helicopters to the spawning option.