If you are playing on the PC using the game launcher you click on the version of the game you want to run that session, one has Horizons in the name, if you are playing via Steam or on a console I wouldn’t know.
If in outfitting you have access to the vehicle hanger or in the shipyard there are options to buy Dolphin or Beluga you are in Horizons.
If you are running Horizons there was an option to start the game at a planetary base the starter ship in that option came with a hanger and SRV, the vehicle, if you didn’t select that option then you have to buy a hanger in outfitting and also equip it with as many SRVs as it can take.