You can define the default folder for new notes in Settings → File
As far as I know, this is not possible - the only way to create hierarchies is to put them in different (or nested) folders.
Yet, as a kind of workaround you could establish a Luhmann-like system of naming your notes:
Say, you have a note called “Note” and then want to write a subnote to this topic: just name your subnote something like “Note1” - you can then further extend your system by naming other notes that continue a certain thought or should be treated as subnotes by naming them “Note1a”, “Note1a1” etc. This way you can simply detect their intended hierarchy by your naming-system and the fact that all of them are stored as neighboring notes in your file explorer.