Apple watch

According to Lock or unlock Apple Watch:

"Unlock Apple Watch when you unlock your iPhone: Open the Apple Watch app on your iPhone, tap My Watch, tap Passcode, then turn on Unlock with iPhone."

I don't understand your question, to be honest? Once you have the watch on your wrist and you have signed into the watch, the watch is unlocked until you take the watch off your wrist. It is either active with an Always on Display or by raising your wrist or rotating the crown and there is no need to unlock something, which is already unlocked.

I am curious to understand why you think such a feature would be helpful? But there is no setting for iPhone to unlock Apple Watch.