How do I make my iPhone 12 Pro ring when wearing an Apple Watch?

Notifications (including incoming phone calls) appear on your Apple Watch or iPhone, but not both. If your iPhone is unlocked, you get notifications on your iPhone instead of your Apple Watch. If your iPhone is locked or asleep, you get notifications on your Apple Watch, unless your Apple Watch is locked. For more info, see:

Notifications on your Apple Watch - Apple Support

You can let Apple know you'd like to see something different here:

Product Feedback - Apple

However, that is the way it was designed to work and I doubt it is going to change.

Sorry, I disagree, the incoming standard phone call is the only notification that should ring on both devices. But not Texts or any other of course.

Make sure that the watch notification for phone is Mirror my iphone, and that the iphone has a valid standard ringtone

Whatsapp or Insta calls may have their own unique problems,

I second that.

My iPhone and my watch are both ringing when a regular phone call comes in and the Mute Switch on the iPhone is not enabled.

As mentioned by LD150, WhatsApp or other 3rd party apps that offer audio calls, will be handled as notifications and not as regular phone calls.

In case this is software related and worked in the past, Force restarting your iPhone and Apple Watch may help.

Force restart iPhone - Apple Support

To force restart your watch, press and hold the Digital Crown and the Side button together for about 10sec, until the Apple logo comes back again.