How do Xiaomi and YU! (CyanogenMod) phones have Google Play Store in India?
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.