Q: How does the USB-C regulate output voltage if voltage supply is 24 volts VDC?

The USB-C conforms to the "USB Power Delivery" specifications. Below is how it works

USB-C (C1): The standard C1 component charging board provides one USB Type-C PD port (60W max) and one USB Type-A QC 3.0 port (18W max) for legacy charging.
It is capable of charging most laptops, phones and other handheld devices.USB-C (C2): This charging board provides one USB Type-C PD port (60 W max), but is manufactured to feature a low-profile on the back of the housing , saving space within the cabinet.Power Output Specifications