You purchase the base game which comes with a starter ship. In the game, you can do missions or do cargo trading to earn in-game money which you can use to buy ships.
You can absolutely use real world cash to purchase additional ships, which funds the development of the game. You don’t have to if you don’t want to. Almost all the ships can be purchased with in-game currency.
The main plus point for purchasing ships with real money is they are acvound bound and will stay with you after a database reset.