The board is powered through the USB port and contains two LDO regulators, one to generate 3.3V for the on-board debugger, and an adjustable LDO regulator for the target microcontroller ATtiny3217 and its peripherals. 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 ATtiny3217 Curiosity Nano.
Figure 1. Power Supply Block Diagram