does this number never change even when user has upgraded Android's version?
Android ID will not be changed.
Open the source code that generate the Ssaid(Android ID) getSsaidSettingLocked based on two parameters: a random ID generated on first use to identify a user ID (in multi-users or work profiles), and the app's signing key. If you update android version, these parameters will not be changed.
If user ID is randomized after a factory reset, the app-specific Android ID should be changed.
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