Changing default calendar to other than Gregorian/Buddhist/Japanese calendars to iOS? (Islamic/Hijri and Persian)

I noticed someones iPhone (iOS 15.4.1) has calendar other than the default calendars, which is "Islamic Umm Alqura" calendar. Although it doesn't even appear in the list (it only shows from the menu before going in to the list.

Also I've seen a discussion similar to this on Persian calendar. (How to add more than Gregorian/Buddhist/J… - Apple Community)

I changed language, region, and every thin in that screen go match their settings and we live in the same are + we both bough iPhone from local stores.

Thanks

Note: The main difference between this & Alternate calendar is that it reflects on all apps. unlike the alternate which only shows on calendar app.