Those were three different cases and each had their ow explanation. I love the great courses lecture series on the black death to get an academic overview without going into deep research.

First of all. The basque country was very isolated. They didn't participate very much in the European economy at the time and didn't have the same trade relationships as many other places in Europe. Basically it is the same reason as that their language is so unique and has survived to this day. See also remote places as Iceland and Finland

Secondly. Milan was a very autocratic monarchy. It was under stricte control by their prince. Therefore when the first reports came that trade ships and wool were bringing in a disease they closed the city. They put very harsh laws in place to limite access in and out of the city. That's how they limited the spread of plague.

The polish question is a lot more complicated and it is worth some interesting research. First of all the borders of Poland at the time were different from what they are today. That is important as when the Plague arrived there it had the same mortality numbers as the rest of Eurasia. It reeked havoc in Gdansk and other Baltic trade cities that were rather a part of the Teutonic states or Lithuania at the time. It is also possible to read in the records in Krakow that one third of the population disappears around the middle of 14th century

It was implied in the great courses lecture series, that during the Soviet era they were furthering the narrative of polish/ Slavic exceptionalism by furthering the narrative that the black death spared Poland and the records on the black death mortality were suppressed to support the narrative. I would like to know more about that

Iceland and Finland and other places that were spared the first waves of the plague got hit later on and had a similar mortality. If Krakow was saved from the first wave , then it got it's first hit in the 16th century when 20% lost their lives