Device manufacturers not just use some private API or SDKs without making deal with Google. Except might be for Chinese noname brands from AliExpress.
All device manufacturers do have agreements with Google about support for their devices and such agreements can very well be for 10+ years or even for lifetime of the devices.
PS: Actually 10 years ago when Windows Phone was a thing Microsoft has tried to make their own YouTube client without agreement with Google and it was killed by Google almost immediately.
Google just dropped all third-party Google Home device support, leaving people with e.g. Lenovo Smart Displays twisting in the wind. It wouldn't be out of character for them to "alter the deal" w.r.t. all those old smart TVs and such.
All device manufacturers do have agreements with Google about support for their devices and such agreements can very well be for 10+ years or even for lifetime of the devices.
PS: Actually 10 years ago when Windows Phone was a thing Microsoft has tried to make their own YouTube client without agreement with Google and it was killed by Google almost immediately.