When you import folders of photos into the Photos.app, you cause the option "Keep folder Organisation".
This way Photos will create a folder for each nested folder, and an album for the folders at the deepest nested folder level.
Alternately you can import each folder separately and use the option "Import to Album".
It is easy to organise your photos in Photos by creating albums or smart albums based on keywords. And the albums can be grouped thematically by enclosing them in folders. Folders can be nested.
For example, the sidebar of my main Photos Library is showing many nested folders, organising the albums for different purposes:
if you have several sets of pictures, that you do not need together in one Photos Library, you could use separate Photos Libraries, for example, one library for teaching, one library for research projects. But only one Photos Library, the system Photos Library, can sync with iCloud Photos and is visible in the Media Browser.
I have written this user tip for Photos on Mojave: New to Photos? Some Considerations on How to design the first Photos Library - Apple Community
Photos on Catalina does no longer have the nice Moments view, but the albums and folders can be used like on Mojave. Photos 5.0 on Catalina may be annoying for an artist, who likes to be in control over how the photos are presented. The "Days" view replacing the "Moments" is now no longer showing all photos taken on that day, but a curated collection of photos, hand-picked by Photos, what photos thinks are your best photos, and cropped how Photos thinks is the best part of the photo. I am rarely pleased by the result. (macOS Catalina - Apple)