Covid restrictions based on originating or connecting locations?

In the case of a flight originating in a place with one set of restrictions imposed by the destination country, but connecting on the same airline through a country with more restrictive rules, is there a general rule as to whether the more restrictive rules will be applied?

I’m specifically interested in Canada to Morocco via Portugal- after Morocco re-opens to international arrivals, hopefully early next month. Morocco has Canada on List A but Portugal on List B, which seems without much basis, but that is their current rule for what I can tell.