Will my 12th-gen motherboard support both DDR4 and DDR5 RAM?

Probably not. Most 12th-gen motherboards are built for either DDR4 or DDR5, not both. A few rare dual-channel boards exist, but standard Z690 or B660 boards are one or the other. Check your board’s RAM slot keying—if it’s DDR4, you can’t use DDR5 without a new motherboard.