Do shared cars show on my other devices, my watch, and in backups?
This answer hasn't been translated yet, so it's shown in English.
Shared cars sync on their own, end-to-end encrypted, to everyone on the car. They aren't part of Device Sync (Cloud Sync, iCloud, or Google Drive), WitnessWork Backup, or backup files from Backup & Restore.
They still reach your other devices: any device that uses the same Buddies identity, such as one you set up with your recovery key or Set Up Another Device, shows your shared cars. That's also how they come back on a new phone.
A device that can't open a shared car yet shows a notice at the top of Mileage: “A shared car isn't ready on this device yet. Your changes to it will be shared once it is.” Your changes there stay on the device until then.
Log Trip with Siri on iPhone and Apple Watch includes shared cars. Siri isn't available on Android, and Wear OS can't log trips.