You can create the maps as “sub maps” and make one big map that references all of them. I hope you (and your player) have enough RAM if you do that …
A better option is to use scene/level streaming. You’re going to want to read through almost all of these links to make it work:
https://docs.unrealengine.com/en-US/…ing/index.html
(That link also has documentation on how to load multiple levels from blueprint or C++, but … I would recommend against it.)