A way to include folders in a library?

[Ramsnet]

In Windows 10, I can only merge folders with the library. I can't just add a folder to a library anymore, like in Windows 7/8. I wonder how can I add folders to a library, because I need multiple folders in a library.

 

[USAFRet]

I don't think you can. What about subfolders off the parent Documents or Pictures...