How do i put meshes together? and cut out parts?

I bought the wooden build pack and I want to make some makeshift buildings from them but I can figure out how to combine them into one mesh

select meshes on level which you to want to make mesh
click file/export selected
Meshes will export as one to FBX format. Then you can edit them in 3d max, maya, etc.

If you want just use them in editor, you can group them - select meshes, right click, group
You also can make blueprint out of them - select meshes, in details panel replace with composed blueprint

I think that is what i need