YouTube iOS in Serbian region
In Serbia iPhone can be legitimately purchased, but YT is not available in that region. Why? What should happen in order to get YT app available, as well as google photos?
Those would be questions for YouTube & Google.
The developer*, in this case, YT & Google, are who decide which App Stores their apps are offered in, not Apple. You need to ask those two developers why they don't offer their apps in your country.
*A government can also demand that specific apps not be offered in their country.
M1 Mac mini, macOS 11.2.2; Watch, series 5 GPS/cellular, watchOS 7.3.1; iPad Pro 2nd Gen, iPadOS 14.4; iPad Air 2, iPadOS 14.4; iPhone 12 Pro, iOS 14.4; TV 4th Gen, tvOS 14.; HomePods, audioOS 14.4 and Airport Express w/ Airplay 2