How to use Bluetooth direct vehicle control
You can register your devices as actual keys to your vehicles, just like your phone can with the official Tesla app. To do that, your car will have to support Tesla keycards - currently that includes all Model 3 and Model Y plus all 2021+ refreshed Model S and Model X vehicles.
To get started, get out your Tesla keycard and go to your car. Then open the app on the device you wish to pair as key - this is important. If you want to pair your watch, perform the pairing directly on the watch, not on your phone. Pairing as key is device specific - but can be repeated for as many devices as you wish to enroll.
From the main screen, tap the Settings button, then scroll down and turn on "Enable Bluetooth direct vehicle control". If prompted to allow the app to use Bluetooth by the device operating system, please allow this, or Bluetooth will not be available for the app to use.
Wait for the vehicle to be found via Bluetooth, then when you see the "Add Key" button in the app, tap it.
Now place your Tesla keycard on the designated area on the vehicle console - on models 3 and Y, this would between the cupholders and the armrest, on S and X it would be on the phone chargers.
A prompt should now show up on the vehicle screen, asking for permission to pair device as key - please confirm this message.
Your devices is now paired as key, and should be immediately connected. You can verify this by going back to the main screen and look for the 🔗 symbol in the top left corner - whenever you see this, it means the app is directly connected to your vehicle, and all compatible actions will be performed via Bluetooth - and will work even while not connected to the internet, as long as within Bluetooth range.
A note about passive entry and walk-away lock.
Please keep in mind that walk-away-lock will not always work, if Bluetooth background processing is terminated by the device operating system. On watchOS this is severely limited, and as such, your vehicle might not lock on it's own, when walking away - in which case you need to remember to lock it yourself.
Background Bluetooth support for passive entry on the Apple Watch requires a series 6+ watch and watchOS 9+. Having a complication from the app on the active watch face helps, as does regularly opening the app - but in practice, watchOS decides by itself how much background processing time each third party app is allowed - so passive entry and walk-away lock might not always work, even on fully supported devices and OS'es.
On thing to keep in mind, if only wearing the watch, and passive entry / walk-away lock is not working, is that Siri is pretty good at reliably opening installed apps. So in case it doesn't work, and you have your hands full, you can always ask Siri to "Open Watch for Tesla", to bring the app to the foreground, in which case Bluetooth should connect instantly.
Not that you might also get a notification from the vehicle itself, that walk-away lock is unavailable - this is normal, and you will get this if your device is not compatible with passive entry - or if background processing time is terminated by your device operating system. Keep this message in mind, and remember to manually lock your vehicle when leaving. This will not affect driving at all.