On flights with no layover (there and back within the same duty period) there are no special requirements as you technically don't enter another country. If it's a trip to somewhere including some stay there, it depends on the rules concerning travel to and from the country in question. You could need a PCR, you might be subject to movement restrictions on either end (i.e. quarantine). An exemption can be obtained under some circumstances allowing the crew to take their rest period on the aircraft in some situations, exempting them from quarantine. That was how most flights with PPE between China and Europe were operated - the crew never left the aircraft in China.