> I wonder how the taxes and fees refund works. Fees I can see airlines trying to say “we already paid these”, but taxes are only charged on services rendered, no? Is there a situation where the airlines have already paid sales tax to the local authority and don’t get a refund for canceled service?

Whether or not the taxes or fees are eligible for a refund to the airline (or don't have to be paid; payment of those will often be delayed) depends on the specific law applicable to each tax or fee, but with the new rule, that's the airline’s issue—they have to refund it to the customer whether or not the airline is still on the hook for it.