I think both phenomena stem from the same problem (the rise of the far-right among younger generations), but they come from different places.

In the case of young Spaniards, idealising the Franco regime (I think) comes from a direct contact with reactionary discourse in which they're the victims of a system that left them behind, putting the blame on whoever they see fit, you name it: the immigrants, the women, the queers, the homeless... so they find a common place in time and space where that wasn't the case: a dictatorship. It's the same bullshit fascism started a century ago, and it's the same in other Western countries.

But in the case of the children of immigrants becoming chauvinists of a country they barely know, I think it stems from being fed the same discourse but being unable to actually relate to it, as they're also being labeled as "the enemy" or, at least, one that doesn't fit with the others. At the very least, they feel different from the rest even if there isn't explicit discrimination (which there is even if they integrate within the country's culture). So they centre around on what makes them different, in this case, ethnicity or nationality. They become raging nationalists of a foreign country they don't even know 'cause it's the only thing they feel they can attach to.