You'll need access to various gaming platforms to trade cross-platform in Rocket League. If you want to trade with a player on Nintendo Switch, you'll need to join into the game from their platform or invite them to trade with you when they're on yours.
You can invite them to your Rocket League party if the "play on the same platform" box is ticked. Select their name or car and pick "Invite to Trade" once they accept.
For the transaction to be executed, both players must confirm it, and an additional countdown will begin, allowing any of the players to cancel the deal if something goes wrong.
If you previously used an Epic account to combine all of your progress across multiple platforms, all of your stuff will be useable when you transfer platforms with your Epic accounts. No matter what gaming device you use to play Rocket League, you'll be able to equip them.