When you purchase the game and/or DLC content for any platform (Steam, PlayStation 4, Xbox One or Nintendo Switch), the game's license and DLC will be directly tied to the account that made the purchase. While on PlayStation 4, Xbox One and Nintendo Switch you may share the content with other accounts that are using the same console, it is important to note that the game's license and DLCs are non-transferable.
Similarly, any content that you have acquired in-game may only be transferred between accounts that are on the same platform, via the Player-to-Player Trading system.
Transferring information across different platforms is not an available feature in Rocket League. Your level, in-game items or DLC content will not carry over to any other platform.