Is this explained somewhere in the manual?
Yup.
I tried to keep up, but I'm still confused. Ha ha. Maybe when I own the car will understand better, but I don't know if I like that when you have to honk, the car will store 10 minutes of video. Hope it's a user setting. By the way, I was told 2021 cars already comes with some sort of USB storage device, but the Tesla guy didn't know the capacity, or how long it can record.
It's pretty simple.
If you have properly formatted storage attached, and dashcam feature is turned on (rectangular icon with red dot showing) then:
The car is recording. Period.
Driving, parked, sentry on or off- doesn't matter- dashcam is recording to the USB storage device recent area. It'll have the most recent hour of footage max after which it overwrites older stuff.
From there:
If you tap the camera icon it'll copy the most recent 10 minutes of recorded footage to the "saved" folder and NOT overwrite that ever.
You can also configure it to do the same thing when you honk the horn (this is an optional setting).
As far as sentry goes- it never "records" anything.
If sentry is off it does nothing.
if sentry is on it does these things:
Goes to alert mode if someone/something approaches the car- this is where it might flash the lights, put up the YOU ARE BEING RECORDED display, etc.
If it goes to alert mode it will attempt to move the most recent 10 minutes that dashcam (not sentry) already recorded into a Sentry folder. This footage is NOT overwritten unless the drive is almost entirely full, then it'll start to overwrite the oldest content here.
If it goes to ALARM mode (ie someone actually is breaking in) it'll also blare max volume classical music and send an alert to your phone via the app.
Note all of the above STILL works on sentry except the moving-most-recent footage bit even if you have no storage device attached.
As to 2021s, they supposedly have a new USB port in the glove box that older cars did not, and a rebranded Samsung (with a Tesla logo) USB key....IIRC it's 64 GB.
The cameras record about 2MB per second, so 120MB per minute, or 7.2 GB per hour.
Allocating for the 1 hour "recent" buffer that overwrites itself that leaves you enough to save (via either the saved or sentry alert methods described above) another nearly 9 hours of video before you're full.