Camera iPhone 12

Yes. If using the rear camera for Portrait mode, tap on the (1x) circle in the corner to change Zoom levels.

link-> Use Portrait mode on your iPhone - Apple Support

The front facing camera has no Zoom option in portrait mode.

Portrait mode on the front facing camera, the one above the screen, has no Zoom option.

Portrait mode using the rear facing cameras (on the back of the iPhone) has 1x or 2x Zoom. You can switch between front and back cameras by tapping on the circular arrows on the bottom right corner.

It is possible it is not available on the iPhone 12. In looking at the specs for the iPhone 12 Pro and the iPhone 12, it is not indicated anything about Zoom for Portrait mode, so it is hard to say unless someone with an iPhone 12 shows up here and can verify the difference.

Its entirely possible thew iPhone 12 does not support the 2x zoom option.

The support document provided above has this to say:

Some iPhone models have multiple options for Portrait mode, like 1x or 2x. Just tap the 1x or 2x icon to switch between the options. With iPhone XR and iPhone SE (2nd generation), the rear-facing camera must detect a person's face to take a Portrait mode photo.

Which means it may not be available in all models.

Yes. Just tap on the 1x or 2x displayed in the lower left corner of the viewfinder.

I know that it is on the iPhone 12 Pro.

You're welcome.