How do two AppleIDs (for two separate countries) work in new 14.3 iOS?

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 wasn't possible), though up to now, it was no big deal to switch between the two AppleIDs on same iPhone.

Now, if I try to exit one of the AppleIDs, since upgrading to 14.3 iOS, 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, but there is no obvious way to communicate 'yes'...is it implicit if I continue?

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.

There is 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 my United Kingdom Apple ID

and I have some apps purchased and available only through my 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.