why do you need wifi to unlock your phone using your watch? if we are outside we are not connected to any wifi but our mobile data!

I don't know that it does use wifi, it just says, "Your iPhone and Apple Watch need to have Wi-Fi and Bluetooth turned on."

Unlock your iPhone with Apple Watch when you're wearing a face mask - Apple Support

I would guess it is just using Bluetooth.

Jason Chew wrote:

why do you need wifi to unlock your phone using your watch? if we are outside we are not connected to any wifi but our mobile data!

I believe this is consistent with Apple's Continuity features.

Bluetooth is used for the 2 devices to "see" each other and set up the feature. The wifi radios in each device, not a wifi signal, is used to communicate directly between the devices. Device to device using wifi.

I disconnected from Wi-Fi, and the feature still works (fortunately). Apparently Wi-Fi just needs to be enabled, and does not need to be connected.

Indeed, that was what I was saying in the quote from Apple. It needs to be enabled but is probably not actually using wi-fi to communicate.

Its the same thing with AirDrop and I'm pretty sure it uses the same style communication.