As I understand it Google Corp originally designed Android so that everyone would use a google account for everything and they could collect our data. So the built-in contacts list was designed to be stored on a Google server along with your Google Mail, your calendar, your notes, your reminders, your bookmarks, your search history, etc etc etc.
I think they realized that some people don’t want that, so they created the possibility to store your contacts and appointments locally, on the device itself.
As long as I have had an Android phone I have had trouble with this. There are always two calendars, two sets of contacts, even tho I never want more than one. It is possible to learn to use only one, set up the same one active all the time, but the basics of the software allow two, whether I like it or not.
e.foundation provides a replacement for the Google services which spy on everybody. Very many people want to synch their calendars to a server, so the /e/ account does that for them. I totally understand why. I am a bit of an outlier, in that I don’t need or want that service. So, I just learn to live with having an extra calendar I don’t want.