Most older doorbells run on 16v AC, newer ones often use 24v AC so that they can support longer runs (probably due to houses being much larger) and there’s a voltage drop as the wires travel farther. It’s perfect that the Wyze units can take the range of 16 to 24v.
They’re all AC - always have been. Often there’s a transformer plugged into an outlet near the furnace / fuse panel or somewhere in the basement/utility area, and the wires that power it come up to the door chime and follow through to the doorbell button on the door. It simply completes the circuit when pressed (like a light switch does) and powers the doorbell. AC transformers are the cheapest to manufacture because they’re simply two coils placed next to each other with different numbers of windings. Probably 5:1 reducing 120v down to 24v or 6:1 down to 20v.
Hope that helps!