iPhone 12 Charging Brick

Yes, the charger is in the phone, and it will only take what it needs from the power source. Both the 20 watt and 61 watt chargers support the USB-C PD (Power Distribution) standard, where the device being charged “negotiates” the voltage and current it “wants” from the power source. That’s why it takes a couple of seconds to start charging when you plug it in.

Yes. You can use any Apple power source and cable that fit without forcing. All Apple charging products are universal and cross-compatible; the only thing that will change is charging time; sources that provide less than 12 watts will charge much slower.