How do Xiaomi and YU! (CyanogenMod) phones have Google Play Store in India? Earlier I had asked a query on how CyanogenMod is it from Android. The answer mentioned an interesting point:


  CyanogenMod is not permitted to ship the Google-Apps with their ROMs,
  you have to install them manually (if you want them), or leave them
  out.


Now phones like Xiaomi and YU! are being launched in India, both running forked version of Android - Xiaomi's own MIUI OS and CyanogenMod respectively.

From the earlier answer and this article I understood that the forked versions of Android will not be Google approved and thus will not be able to access Google Play Store. But both these phones have access to Google Play Store and come pre-installed with them. How and why?

Also why does Amazon's Fire OS still is unable to access Google Play Store (hope I am right on this one)?
Core Android ROMs have pre-installed google apps.  Nexus mobiles have Core Android ROM.
Nowadays each manufacturers have their own custom roms such as Lg has its own  os(Forked versions of Android) same as Xiaomi(MIUI). Samsung , Sony , Micromax have also started to customize their OS in order to achieve better performance.

Google apps needed only plateform Android. Custom Roms make no difference to installation and support of google apps. Each android custom roms support all google apps. Sometimes they have given pre-installed google apps ans sometimes not. At that time we just need to download it.