Handy tip on how to nest folders in Apple's Notes app. Unfortunately the nesting has to be done on a Mac, but it does then carry over to Notes on iOS.
Open Apple Notes on your Mac and simply drag an existing folder on top of another. That adds a disclosure triangle to the folder and placed the moved folder inside the destination folder. Then open up the Apple Notes application on your iPhone or iPad and give it a second to synchronize and you’re good to go.