Let's start with some basics:
Importing, Genres, Artists, Albums, etc: MC reads the tags *inside* the files to figure out all of this stuff. You might see these called ID3 tags, or metadata, or just "tags". Either way, this stuff is embedded inside each song. It gets set when you rip it. So in your case, DBPoweramp put the tags there. They can be changed. But JRiver MC reads whatever tags are there. This is where the Genre is coming from: inside the file, in the tags that were previously written.
MC's presentation of tags and metadata: MC doesn't exactly have "folders" in it's interface. It kinda does, but that's missing the point. The point is the MC uses Views. Views can be set up to present songs/artists/genres/albums in many different ways. For example, in the left pane, under Audio, you'll see Albums. That view shows you all of your albums, by album name. Under Audio you'll also see Artists. This view shows you artists first. Then when you double click an Artist name, it shows you all of the Albums by that Artist. You can set up new views if you want to that present things in the way that YOU want to see them. Genre and subgenre can be used in these views if you choose to. Again, note that these views are driven by the Metadata TAGS. Not by the folder structure. This is an advantage, not a disadvantage. Keep reading...
Assigning metadata tags yourself: You can edit the metadata tags (like Genre) any time you want. Edit > Tag will bring up a window on the left with tags inside the file. You can edit any of them you want. You can alter the Genre for a whole album at once (for example).
Let's get to something specific to tie this together:
Newbie here. I hope to set up my library hierarchically to mirror the tree structure of music that resides a folder on my laptop. The first level down are genre folders, the second level is usually subgenre folders, the third level is a folder labeled by composer or performer. For example:
Music/Jazz/female voice/melody gardot/albums/tracks
Music/Jazz/female voice/billie holiday/albums/tracks
Music/Classical/ Saint-Saëns/Symphony No. 3 [Munch]/tracks
MC has the ability to read folder and file names and assign tags based on what it sees there. This is done in a tool located at: Tools > Library Tools > Fill Properties from Filename
As an example, you could highlight some Jazz files and then open the Fill Properties from filename tool and type this into the template:
Music/[Genre]/[Style]/stuff/stuff
MC would then fill in Genre and Style for you, based on the folder names you have used. You could do this for many files at once, so it wouldn't take you a long time or anything.
You would want to experiment with just a few files at first and see if this worked as you expected, then move on to doing more. But you'll want to play around with Views at the same time and create views that work for you. Hopefully organizing by Genre and Style will work for you. MC has the flexibility to organize almost any way you want to. You can even create your own fields. Many people do so and it's not hard.
So is there a way to import from the top level so that (my) genres and (my) subgenres are created automatically to match the file structure?
The way I outlined above could be used on a lot of files at once. Just so long as they matched the template. You might want to do classical separately because it's folder structure is different. Any other genres that have a different folder structure should probably be done separately also.
Once you get this set up and running, adding new CDs should be easy. You can just import them and tag them correctly as you import them, using the Tagging window. Or you could set up auto import rules to do it more automatically. But that's sort of advanced.
MC is a big mature product that can do a lot. Take it step by step. Ask questions. You'll get what you want if you put in some effort. Good luck!
Brian.