The kit is powered through the USB port and contains two regulators for generating 3.3V for the debugger and an adjustable regulator for the target. The voltage from the USB connector can vary between 4.4V to 5.25V (according to the USB specification) and will limit the maximum voltage to the target. The figure below shows the entire power supply system on ATmega4809 Curiosity Nano.
Figure 1. Power Supply Block Diagram