https://store.arduino.cc/arduino-uno-rev3

Microcontroller: ATmega328P
IO voltage: 5V

IO pin high voltage (for VCC = 5V): min 0.6VCC to max VCC + 0.5
So digital IO pins have a nominal high level input range of 3.0V to 5.5V, so are compatible with 3.3V input signals.

UART pins

RX and TX are marked as the pins that are RX and TX on the remote device!!! They are opposite to the actual RX and TX pins in the Arduino Uno itself)

Arduino	Function
IO0	Arduino RX (remote device TX)
IO1	Arduino TX (remote device RX)