The Model 3 Manual under "TeslaCam" should help you get the basics down. You'll want a USB flashdrive that has some decent file space as TeslaCam and Sentry Mode use 3 cameras now. TeslaCam will record and store 1-hour of video (1 min. each for each of the 3 cameras). 1-minute videos are about 30MB. The TeslaCam's 1-hr buffer of live video will write over itself. You will need to manually create a folder on the flashdrive's root level called TeslaCam. The temporary videos from this buffer are stored inside the TeslaCam folder inside a folder called "RecentClips". This folder will be created automatically. The buffer is apx. 5.4GB (3 cameras x 60 min x 30MB each = 5400MB).
Saved video -- "SavedClips" folder
While are using TeslaCam, live footage can be saved off into it's own file folder called "SavedClips". This folder is created automatically. Anytime something is "saved", it will consist of a file folder date/time stamped with 10-minutes worth of 1-minute clips from all 3 camera--front and left and right repeaters. Again each minute is apx. 30MB, so each folder will contain about 900MB or .9GB of video.
Sentry Mode is used during parking and currently needs to be turned on each time you want to use it. It will automatically save a date/time stamped file to the SavedClips folder when it detects movement (people/lights or loud sound/vibration around your car--best we've determined here on TMC). Car must be in Park. Sentry Mode can be sensitive, and will save off files which can fill up your drive quickly while parked in a busy parking lot or parking garage for example. The important thing to remember is that video in SavedClips DO NOT write over themselves. You will need to manually do housekeeping on your flashdrive and purge the files you don't need.
Another note mentioned in the Manual on TeslaCam -- make sure you Pause the USB drive (see directions) before you remove the drive. It needs to properly shut down or you can corrupt the flashdrive.
If you fill up your flashdrive, recording will stop and you will see a gray X over the camera icon on your center screen. A number of owners have encountered this with smaller flashdrives in a very short amount of time using both TeslaCam and Sentry Mode.
You can view the USB drive video files on your computer using the USB connector. You will need a computer to reformat your USB drive and do file maintenance. There is a USB drive out there that has a lightning connector you can plug into your iPhone. Some USB drives have a USB-C connector end as well. There isn't a way to watch the video files on your center screen.
More to learn but hope this gets you started.