I did more research on Nextcloud and am posting here in case other newbies can use the info. Hopefully, I will get the technicals correct.
Nextcloud servers do not run on Windows, so forget that idea. Nextcloud can run on StartOS because StartOS is Linux under the hood. The Nextcloud client can run on Windows. Its job is to act as a middleman between Windows and the NC server on Linux. The NC client associates a folder on the server with a local folder visible in the Windows file manager. The folder is typically called “Nextcloud” like OneDrive or Dropbox.
The NC client requires a normal (non-onion, non-Tor) https://NC-server address to connect to the server. Here is where things diverge into different scenarios because StartOS is Linux and runs on Tor using onion addresses.
StartOS on the Local Network
If your StartOS box is on your local network (mine is), then you can give the NC client a .local Tor address like “https://gibberish.local.” But that works ONLY if you have installed on your Windows machine the ancient Bonjour printer address software from Apple. The NC client passes the .local address to the system, the system calls the Bonjour server to resolves the gibberish.local address into an IP address on the local net, and the system hands the local IP address (like 192.168.1.25) back to the NC client. The NC client then contacts the StartOS NC server and starts synchronizing files.
StartOS on a Remote Network (with my friend’s Windows machine and NC client calling)
When my non-local friend tries to contact the NC server on my StartOS box, he configures his web browser to use Tor onion addresses. Then the browser runs over Tor, contacts my NC server, and logs in. No problem. He is using Tor, and my StartOS box is using Tor. But my friend must use the web interface to NC, which is clunky.
If my friend tries to use the NC client on his Windows machine, what address can he give to the client for the http address of the server? If he gives the client “https://gibberish.onion,” the NC client hands .onion to his Windows OS, and Windows cannot resolve the onion address. Failure. If he gives “https://gibberish.local” to the client, that fails too. My friend does not have Bonjour installed on his machine and does not have my NC server on his local network. So, there would be no point in having Bonjour installed on his machine unless he had his own NC server running on a StartOS box on his home network.
If the Nextcloud Client was smart about Tor
The NC client on my friend’s machine needs some way to convert my server’s onion address into an IP address that resolves to the NC server on my StartOS box. The nearest thing (which does not work) would be if the NC client allowed a software proxy to be used in its network settings. Then perhaps the NC client could use a Tor proxy Windows service (available in the expert package from TorProject.org) to connect to my onion StartOS box.
The Tor service is an application-level proxy for socket connections, and socket proxies are used by specific applications. They are not systems-level interfaces used for all socket communications from the computer. That is why you can (must) set up each one of your individual browsers to use Tor proxies instead of setting up your whole computer to use only Tor connections (which would be a bad thing, obviously - you want the flexibility at the application level to use Tor or not).
In the case of the Nextcloud client, it is not yet smart enough to offer configuration options to use Tor for some folder-server associations. It might never be that smart.
The bottom line is that non-local Windows users will probably never be able to use a Nextcloud client to integrate with the Windows file manager. They will always be forced to use the web interface to reach a Nextcloud server running on a Tor-only StartOS machine until StartOS offers clearnet (non-Tor) support (maybe sometime next year).
I do not know enough to opine on whether it would be possible to run some software servers on StartOS over Tor (onion addresses) while running other services using normal .com URLs. Maybe the whole StartOS machine must run Tor or clearnet addresses. Maybe someone else can comment on this issue.
I think I got the technicals correct here, but if I did not, I can fix the errors if someone points them out.