On Frieza Force space ships, I assume. There were like what...maybe five Saiyans as part of the Frieza Force after Frieza blew their planet up. No need for them to have a planet.

Not a planet but they still need an appartment. And if there is no appartment available you pay Frieza with your work wage to kill the previous tenants.

Like there is only two people strong enough to not just get invaded by frieza and it's buu and beerus. There is no one to sell the planets to except his own employees, no economy except the direct production made to the empire, the planet auctions are basically company performance bids whoever genocided fast enough or made the pod ships quota get a new planet