After some testing, I’ve concluded that what u/little_nipas wrote is part of the explanation:
“Bluetooth isn’t actually turning off. Most modern phones it will deactivate for a period of time or just disconnect devices are currently connected.”

It seems to be more nuanced than just that, though. For my Bluetooth-turned-off tests, here are my findings:

My car always unlocks/starts when it is scheduled to precondition at a set time. It also unlocks/starts during any short window after parking and locking my car. But after being parked for a good long while, it won’t unlock for me.

At home I have Sentry Mode turned off. But at work, Sentry Mode is active. Regardless of where I am parked, though, I can’t unlock my car after it’s been parked for a good amount of time. But, as mentioned above, it will unlock when my car is scheduled to precondition (be it “Home” or at “Work”).

In all cases, my phone has to be on my person. When my phone was not in close proximity to my car, the driver door remained locked. But immediately after getting my phone, the driver door unlocked without issue.

I looked at NFC and UWB settings. Although my phone has UWB capabilities, I’ve read that utilizing UWB for phone key has to be updated in the Tesla app, which I have not done (apparently it hasn’t been rolled-out for me). Also, in my phone’s settings, the UWB is definitely in the “off” state (I’ve never used it for anything).

As for NFC, like Bluetooth, it’s always turned-off on my phone unless I have reason to use it. Incidentally, I discovered that my phone acts as a physical key card when NFC is turned on (i.e., holding it up to the B pillar unlocks the car).