Clarification: Most Android users don’t use custom firmware (“ROM”). OEM means the company that makes your phone. All the major ones use Google Play Services. Nothing and OnePlus might be exceptions as those are known for using custom firmware that is more private. Also, Pixel phones running GrapheneOS ostensibly do not include Google Play Services. But Pixel phones don’t ship with GrapheneOS. You have to add that yourself by unlocking the bootloader (available deep in the settings, wipes the phone), and manually flashing the custom firmware (CFW or “ROM”) to the phone.