How do i merge my projects?

I have my main project which is my terrain level,and i have several other projects, which are buildings all made using ue4, i would now like to take my buildings and place them in my terrain level, how is it possible to do this? can anyone help ?

Right click on the asset in the content browser, select asset actions, select migrate (or select an entire folder and select migrate). Choose the content browser path in the project you are moving the assets to. It will migrate all the assets’ dependencies as well. You’ll want to migrate the other assets into the one with the landscape. If I am correct, you cannot migrate the landscape by itself, only export it. You could, however, export the height map of your landscape and add it to a landscape in the other project, or migrate the map containing the landscape.

Migrate assets docs