They are a for-profit company. They are closed-sourced both in hardware and software. Their ecosystem is known to be a walled garden. They aren't open in their processes. They sell your data, they just don't tell you. Following all of their ToS you have to agree to use their devices - you don't even own anything, you are just paying for a subscription (that can be revoked any time for any reason) to use the device.

I agree with most of what you said. But how do they sell their customers’ data?

You can't know, it's kept in a secret. But given the input data - you just have to assume they do (even if they don't).