Photoshop :: Put New File Into An Open File- In Its Own Layer

I have an open file - we'll call it the 1st file. I will add a background and some other images to it that are located in other psd files. But how? How do I get them in there?

I want to add 2 images, (don't worry about getting into selection and pixels, shadows, edges, etc. the complex stuff is easy LOL) Anyhow, I have another file that I open. How do I get that entire image(or file) into the 1st file - into it's own layer - so I can edit, position, and transform it etc. as I need.

make sense?

What if there's a 3rd image/file that I want to open and add to yet another layer.

Do I simply select the entire image and copy/paste it? I tried dragging it, knowing that would probably be futile. Anyhow, any and ALL advice on this surely simple procedure would be appreciated.

There are some files with at least 8 files that I want to bring together into one image. Same thing, do I cut and paste them, or is there another way?

All of these are psd files, btw. Would it make a difference if one was different, say a jpeg or tiff etc.