Turn a feed on and events from that place show up here automatically. Turn it off and they disappear again — nothing is copied, so nothing is left behind.
The places that can feed this calendar are the ones listed here, and they are all inside this app. An outside calendar — Google, Outlook, Apple — cannot be connected, so there is no feed to add for one. To bring its events in, export it and paste the file under Import events below.
These send events out to My Church rather than bringing them in, so there is nothing here to turn on. Which events appear is set on each event, with Publish to My Church.