it has a rearview mirror to satisfy DOT rules but you cant see anything out of it and by that i mean literally nothing you just see the back seat of your cabin you need the screen for a rearview
im almost certain thats what they are. like the rules for side mirrors cover the view angle and positioning and such, so for instance a drivers side mirror cannot be concave or convex it must be a flat mirror but passenger side does not (which is why it has the warning about objects appearing closer than they appear), but i dont think that it ever states they need to be unobstructed
this is frustrating for motorcyclists customizing their bikes because (at least this used to be the case) those little round mirrors people would put on the end of their handle bars to replace the bunny ear mirrors on their motorcycles are technically illegal. even though they often function better than the legal ones because they give a better rear view by their nature (and no good motorcyclist would ever merge lanes without turning a little and looking first anyways)
Tesla found it more convenient to use the rear camera to display rear traffic on the center screen than to install a rearview mirror in the cabin. Not that the Cybertruck doesn't come with a cabin rearview mirror. It does, although it's removable. This makes sense, considering that you cannot use it to see outside when the vault is closed. However, this approach poses a critical problem that has nothing futuristic about it.
As teardown veteran Sandy Munro discovered immediately after delivery, when he tested the Cybertruck, driving in the rain is incompatible with the rearview camera approach. Thanks to its shape and aerodynamics, the airflow around the truck creates a vortex, directing the dirt toward the rear camera. It may have eaves to protect it from the water droplets coming vertically, but Cybertruck designers did not think of the dirt drawn by the airflow when driving in the rain