If you're talking about the L/R rearview (external) mirrors, they snap on. I only know because after owning mine for a couple weeks I walked up and the passenger mirror cap had come disconnected, leaving a half-inch gap between the painted part and the black. I'm not sure how it happened, but it snapped back into place easily and has not moved since. I didn't get the feeling that it wouldn't happen again.
Rivian doesn’t publish service manual's so very unlikely for owners to know. I wish they did, but understand why they wouldn’t. Those who work for body shops and salvage yards might, but you probably won’t find any of them here. Might just have to buy a salvage unit online and do an autopsy. Either snapped on or screwed on with access to screws once mirror is popped off (usually how most housing are put together).