From the research I've been doing on this, enabling UWB on the iPhone is thru Settings | Privacy & Security | Location Services | System Services | Networking & Wireless. If that's green, UWB is on. It's a little weird to me, but that's what I read from a few sites.
To see if you've used UWB for your Tesla, another site said to navigate to Settings | Privacy & Security | Nearby Interactions. They indicate you're supposed to see something related to Tesla there, but I don't know what you're supposed to see. I also do not know for certain if you're supposed to be standing near your Tesla for this to work or not.
Unfortunately I have a 2023MY and it does not have UWB in it so none of this applies to me. Hopefully it helps you.