'Lego Fortnite' is a persistent mode, you can create new sandboxes and play through the quests and progress through 3 biomes at any time. The events they then bring to the game are available in those games. So for example the for the Star Wars event, this happens in-game:
... then a rebel comes to your camp and asks for help, and gives you some binoculars so you can survey and see where rebel and imperial signals are. The first one takes you into and through a cave into a fresh map to then build up a rebel base. Caves normally just spit you out on another part of the map, so this is how they're implementing new content and linking you to a new map, which is kinda cool. Progression of that base is similar to the base Lego Fortnite, but all Star Wars themed with a lot of new stuff.
Basically a lot of their IP integrations going forward will probably do similar things.
Overall it's relatively limited, but we had fun with it for a number of hours at a LAN, got through the first couple biomes, took on a couple of the big roaming monsters. I don't play much Fortnite outside of this stuff, but hoping I get far enough into the battle pass this season to unlock the Fallout armor stuff (usable for other modes I rarely play - Rock Band and Battle Royale). Too bad there's no Lego version of it (yet?).
Of course, youtube is full of people doing crazy shit in this mode.