PhotoShop-How to add a 2nd file into a layer of the 1st file so I can combine them and as one file?

@Conrad_C mentioned placing as Smart Objects already, but I would like to definitely recommend working with those. 

If it becomes necessary to scale, transform, … an object/layer multiple times in the course of the layout process plain pixel layers can accumulate degredation whereas Smart Object-instances are calculated based on the original data. 

If the first file is open but the second is not, you could simply drag the second file from the desktop and drop it inside the Photoshop document window. (The command equivalent is choosing File > Place Embedded to import the second file.)

If both files are open in Photoshop, you can drag any layer from the second file and drop it into the document window of the first file. This works if the document windows/tabs are open side by side, or you can drag a layer and hold it over the top of the other document’s tab until that window comes forward, then drop.