What about countries like China, most of their developers won't be registering with google ne. They don't even distribute their apps through play stores
It does not matter, they will do the validation through the play services, If the manufacturer wants to use google services, they will have to comply.
Google already has APIs to validate if the app is from legitimate sources (Google play integrity services), but that needs an implementation from the developer. It looks like the google is trying to do this without needing any changes from the developer.
Europe wala iOS walatath third party apps install karanna denna una. Google ekatath Europe wala nam oka karanna amaru wei.
Nope,
EU rule is to allow 3rd party store or other installation options (without Apple's App Store).. But the developer has to be a registered apple developer, in fact the eveloper has to go through apple and their approval process, then only the developer is able to destribute the app through their website/3rd party store.
https://developer.apple.com/documentation/marketplacekit/distributing-your-app-from-your-website
(iOS app has to be signed by apple to run on iOS device, regardless your destribution method, the app has to signed by apple, this signing mechansim is very diffuclut to bypass)
Android also needs a signed app to run, and that can be disabled, but google can do a similar implementation to apple, then if you want to run an APK from an un-verfied source, you will have to use a custom rom. The problem is lot of manfacturers do not allow to unlock bootloader and flash custom roms, and even if it is possible what % of users are going to do that?
Open source නම් එහෙම කරන්න පුළුවන් ද?? 
Yes.
Open source means source is open to anyone. However, current google version of the Android is not fully open source. It consists of many closed source components. Also open source code is not up-to date, google keeping the latest code private, so the open source code is not upto date now.
This is very similar to the google chrome situation. Google chrome is not open source(contray to the popular belief), If you want the open source version, you have to use Chromium or try to build from the soruce, very few people are doing this and vast majority is using Google's closed source version, thinking it is open source.
iPhone හොදයි වගේ
