They're moving away from Apple because Apple is the one company with enough power to force them to respect customers' privacy and give them choice, with App Store privacy disclosure requirements and App Tracking Transparency. If Apple's focus and attention on privacy-conscious technology continues to gain traction, it may shut the door on society's acceptance of the mass data collection apparatus that companies like Google, Amazon, and Facebook/Meta have profited off of for nearly two decades.
GM wants to build their own subscription model for the in-car informtainment experience because they aren't satisfied enough with making profit on the sale of vehicles and ongoing service. Google doesn't care. They'll gladly cooperate to get access to millions of unsuspecting drivers, and collect your driving habits and personal data, which is insanely profitable to them at your expense. GM gets to keep the subscription profits. It's a match made in hell.
Same for Facebook, they just want to be on platforms that don't care what data they collect and what they do with it. Google has no problem being that enabler.
You're right that these companies believe Apple is getting too powerful and dominant. But specifically, they fear Apple's dominance over the market that is encouraging a pro-consumer direction in terms of personal data privacy that will lock them out of collecting your data without your explicit consent, forcing them to render services without you being forced to share unnecessary data with them, or giving you control back over how that data is collected, shared with other parties, and deleted when you want it to be.
