Nothing prevents you from using Samsung, Xiaomi or Huawei stores and cloud services, Google Play is only the main store if you configure it as such.

Google SDK naturally doesn't have the support for OEMs specific features.

Leaving just one Samsung example here, too lazy to dump everything else,

https://developer.samsung.com/mobile/galaxy-sdk-getting-star...

I am yet to see anyone disable OneUI, MIUI, HyperOS, EMUI for bare bones AOSP UI.