Needless to say, the potential at your fingertips when you figure it out is only limited by what you can think up. If you have an idea, chase it (or check if it has already been made by Lethamyr).
If you just want to get into all those great maps, you've come to the right place. This is how to get custom maps in Rocket League. Unfortunately, this doesn't work on Xbox, PlayStation or Switch but works great on PC.
Rocket League Custom Map Tutorial
First, you need to download the custom loader from this link.
Make a folder and call it Map Files. Once the link has downloaded, run it and find the map folder when it tells you to "Load your map directory."
After this, it will ask you to locate your Rocket League Installation Folder. Mine was just (c:)/program files/epic games/rocketleague. If yours isn't there, the little question mark on the popup can help you.
From here, simply find a map you like, download it, and import it with the custom map loader app. To play your custom map, get it ready in the loader and just open up Underpass in Freeplay or a Lan match to get going.
Moving Steam Workshop Maps Into the Epic Games Store
Whilst you now have access to some of the greatest maps out there, you're still missing the Steam Workshop maps. All you have to do is grab the link to the map and paste it into this site.
You can simply download the map, import it in, and you're all ready to go.