I have no support/technical question and have seen the support category. (Be aware that direct support questions will be deleted.)
on
Which general topic do you have
I had a confusing time with importing a URL calendar so was hoping to make someone elseās life easier in the future.
I wanted to subscribe to a webcal:// calendar online, but that didnāt work out. I then downloaded it with wget, substituting webcal:// with https://. This at least worked, so I gave the saved file a .ics extension and imported it via Calendar settings. I canāt remember if there had been a problem or not, but I can also say the .vcs extension worked. This is simply āVCalendarā, an older version of the āiCalā format. I had this sort of working on Google Calendar, but the events would play a very nasty trick of disappearing the moment they were most needed, so I guess you canāt go wrong with downloading: plus once imported one-time in a file they become writable, allowing you to make adjustments (which in theory are supposed to come from the system managing the calendar, but in practice thatās usually left to us humble users XD).
It feels nice to have Nextcloud manage the complexity, as once itās set up there, linking each device only takes a webcal:// link pointing to your cloud instance, as all the calendars and their configurations are managed collectively by Nextcloud, the āgo-betweenā of the situation. Adding calendars on different apps on different OSes, on the other hand, can be stressful due to device-specific quirks, which greatly increase chance of error compared to the former method.
Since calendars tend to be pesky to set up, perhaps this thread can be continued with more advice on calendar setup 
[UPDATE]
I had even tried running an open source VCalendar to iCal converter, but eventually the solution I give above was what worked. The software I was using was made a decade ago and was struggling with parts of the file, so may need a few tweaks.