How do I combine many small layouts into one big layout?

I am building a game that will have many levels. To speed the level design process up I have decided to build 50 or so smaller layouts and then combine these layouts to make 50 large levels (almost like placing really big tiles). Could someone point me in the right direction on how to do this?

Can you clarify on what you mean when you say combining the layouts?

You can't merge them into one layout in the editor if that's what you mean.

I want it to be like a big tilemap. Imagine it like a tunnel. As an example, I would make a horizontal tunnel piece, a vertical tunnel piece, and a tunnel intersection piece. All of these pieces would be big enough for the player to move around in. I could then drag these pieces together into a level layout and make the level that way. I got this idea from the making of Oblivion video: https://youtu.be/zvm0CN3tQFI?t=7m. In this clip the level designer drags pieces together like a Lego set and makes a whole dungeon out of it. If i could do something similar to that it would speed up the design of the game by a lot.

Are you talking about once you reach the edge of one layout it loads the next layout?