@Jan186 Check this article for more information: https://www.airbnb.com/resources/hosting-homes/a/your-guest-or-host-may-have-covid-19-now-what-168
The answer to (1) is that if any of the guests report to Airbnb that they tested positive, your cancellation policy does not apply, and they'll be able to cancel for a full refund. They might be asked to supply evidence, but there's no info available about how or whether it will be verified.
As for (2), check your local laws. Some places may require licensed accommodation providers to extend a stay for a guest in mandatory quarantine, but Airbnb doesn't have the authority to force a host to do anything. As described in the article above, Airbnb may suspend your listing and cancel upcoming bookings at your expense if a person with Covid has been reported as staying there.
On these concerns, the same policy applies regardless of whether the guests are domestic or international travelers.