Two AppleIDs - since upgrade to 14.3 on iPhone, I'm afraid to exit out of selected AppleID as I get confusing message about data deletion

Since I upgraded my iPhone to 14.3 version, switching between AppleIDs seems to work very differently.

I was forced to have two AppleIDs, one for each country, I would have liked to only have one.

But it was no big deal to switch between the two on same iPhone.

Now, if I try to exit one of the AppleIDs, it gives me a conflicting message, and I don't have the courage to exit after all.

The message I get (translated from Italian) is:

Big letters: "Do you want to keep a copy of your data on iPhone?" My answer: Yes, I do.

Medium sized letters: "The data removed from the iPhone will still be available on iCloud." My thoughts: Hmm, I don't have iCloud activated, and I don't want to activate it.

It has a selection button: "Keep a copy of the data of: Keychain" which either is selected or unselected by choice. My thoughts: I don't understand what would happen if I select it, or what would happen if I didn't.

And finally, in really small letters: "All documents and data stored on iCloud Drive will be deleted from iPhone"

I really don't understand any of this...I'm afraid I'll loose data if I log out...

I have some apps purchased and available only through uk apple id

and I have some apps purchased and available only through italian apple id.

What exactly would happen if I exited the Italian apple id and logged into the uk apple id...

How does the new system work exactly? I'd like to know before I attempt it.