Hi.
I am new to K-9 Android app and installed it as I read it would eventual turn into Thunderbird and really do not like the stock Samsung email client.
Now I see that my multitude of folders subfolders are not nested, but in this format
“aa.bb.cc.dd” etc.
That raises the question. is it a simple configuration that I missed or is that just the way it is? I am hoping for the first option. 
Thx in advance.
It depends in you MTA or server IMAP configuration. Many of them do not actually store subfolders (nesting depth 1) and just pretend to do so. They then store your “subfolders” as specially named folders as in your example.
You can test this on your server by creating a folder and naming it corresponding to the naming scheme (i.e., use the special character). If under Thunderbirds it is displayed as a subfolder, you have your answer.
Not yet supported but a featur request exists on Github for it since ling. Also see here: