There are at least two different keyfobs. I looked at used Model X ones and they apparently don't work on M3. So it's $175 for a new one direct from Tesla. It works just like a smartphone, without having to watch the battery status. There's also a $18 keychain thingy. Without the thingy the keyfob has no way to attach to other keys.
There are also rings and other jewelry constructed with the working part of the card key. I guess they buy cardkeys, dissolve the cardkey material in acetic acid, and weave the remaining chip and antenna into a piece of jewelry.
I printed the owner's manual (284 pages) and have gotten maybe 20% of the way through it. Way too complex for the mass market. The Model 2 is going to need another introductory manual for non-techies. My wife hasn't got her nerve up enough yet to even sit in the driver's seat. My announcing my latest discoveries doesn't encourage her. One thing she really has on me is fastening the seatbelt. She has no problem with it in the passenger seat. Takes me a major battle to attach it. I somehow get the belt in the way of the latch. Other people don't seem to have the problem, so I'll probably eventually get the hang of it.