You can get usb-c to c charging cables that tell you the wattage on the cable, no way in iOS itself I know of.

Be aware that charging speed will vary on all devices in accordance with what the battery controller thinks is best at any given time.

I'm not aware of a way to do that with software. You can buy one of those USB meters for about $20, they tell you how much power is moving through. Something like this (though I'm not sure if this exact model supports USB-PD):

On MacOS Apple provides a command line tool that outputs pages and pages of data on the current power state of various components. The numbers fluctuate wildly from one second to the next.

Right now, my Mac laptop is drawing zero power as I type this, because the battery is fully charged and it's better if the battery doesn't stay fully charged... So the even though my Mac is plugged in it's running off battery power right now to prolong the life of the battery. In a few minutes, it'll flip over to charging the battery, then once it's full it'll swap back to draining.

Apple doesn't provide detailed battery information on iPhone - deliberately because it could be used to perform "fingerprint" tracking. Third party tests (with hardware, not software) have found the iPhone 15 Pro will draw up to 27 watts. But where it gets more complex is that power wouldn't all be going to the battery.

There are apps that workaround the limited power status API, for example by measuring your battery level over ten minutes, but they're not accurate. In fact I think they're less accurate than they used to be, because Apple has reduced the accuracy of the API that provides battery information (again, deliberately, because ad networks were using it to invade user privacy).

How much power the phone draws will depend on the charge level of the battery and the temperature of the battery, as well as obviously the cable and charger that you use. In some situations an iPhone will display a "not charging" status even when it's plugged in. That doesn't mean it's drawing no power at all from the USB cable, it just means it's drawing less power than the phone is currently consuming. Usually if you let the phone cool down a bit it will start charging. Or if you turn the screen off, that might reduce power consumption enough that it will charge slowly.

No it doesn’t. Both the regular and pro lines support 20W max charging.