Noob; how do you merge different blend. files?

Hi, I’m fairly new to Blender, but i think I’m coming on okay.
One problem I’ve run up against recently is that I’ve modeled two buildings, or two parts of a building, separately and saved them as individual files. I now want to combine the two parts to finish the building, but I can’t figure out how to ‘cut’ or ‘copy’ a mesh out of one window, and ‘paste’ it in another open window to combine the two.
I know there are probably ways i could have made this easier on myself, but like I said, I’m a definite noob, and my computer’s a bit low on memory.
Appreciate any help

Append the one mesh to the other file. Just hit Shift+F1 and find your file, then go to mesh, and find your mesh that you want. And then you have to add something like a plane and then in the edit mode buttons, in the link and materials panel, you you will see ME:Plane. Click the arrows to the right and find the name of your mesh.

It really helps if you name the mesh you want something you can find easily. Cube.002 is very hard to find, but building is much easier. Hope this all helps!

It worked! thanks so much!