Hopefully this might help. I am new to PSE 2018 and still learning it but was fairly confident with a much earlier version. Each time you bring in a new file, it becomes the active one in the workspace. To see your new document and other additional files you want to put into it you have to make sure the "Photo Bin" is active so you can view all the open files. Click on the "Photo Bin" icon in the lower left corner.
With the Photo Bin open - click on your new document to bring it to the front on the workspace. It will have a blue outline around it in the Photo Bin.
Any time you use a tool the photo bin closes and the Tool Options appear. Once you are done using the tool, you have to reopen the Photo Bin by clicking on the icon so that you can bring in additional items into your layout.
To bring files into your layout - have the photo bin open - and your layout active in the workspace. Make sure the
Move tool is active. Scroll through the photo bin to find the file you want and grab it (a hand icon will hover over it There is a difference about "Grabbing" and accidently clicking on it which will make that file the active one in the workspace.) Grab the file you want and drag it upwards onto the Layout. It takes a lttle time to load. It often doesn't end up where you want it in the layers palette but you can drag the layer to where you want it in the layer palette. This is one big difference from the earlier version I used and I haven't yet figured out the trick getting a file to load on the layer location I want. The Layer Palette shows all the files that are loaded onto your layout. It is usually located at the right side. If it isn't visible, click on the Layers icon at the bottom of the program window.
Hopefully, this gets you a little farther along with learning and experimenting. I don't often explain things very well and I am still also pretty new to this version of PSE. If the new file you bring onto the layout goes to the wrong position in the layer palette have the move tool active and then grab the thumbnail in the layer palette and move it up or down to where it needs to be.