Older app version, and new app update. Could someone explain this to me or refer me to an appropriate source please.

I have an old app, that is running even on old iOS11 at the moment. This poses a problem for me:

1. I do not want to cut off all the people that rely on the app.

but,

2. I want to upgrade to SwiftUI and use all the new possibilities that iOS14 brings.

What should I do in this situation?

If I release an update supporting only users having iOS14+ what happens to older users and people who are using older systems?

Will they still be able to use the older version, or download it from the app store?

Or I am technically cutting them off.