I found the documentation for using External storage (Local) very helpful, except where it talks about the ownership of the files in the external folder tree.
The documentation for a standard installation says the files should belong to the user under which the web server runs, www-data, but for the snap installation, they need to belong to root, as specified here.
It took me a lot of time and frustration and bothering generous community members to find that out. (See this conversation.) It would help us newcomers to NC if the managers of the standard documentation could add something like this near the top of the page:
If your NextCloud was installed as a snap, the details are different. Please see this document as well.