> I still don't understand how Portuguese and Spanish speakers can understand each other.

I bet that without a relatively large amount of exposure they can’t and then suddenly it’s really obvious what the other person is saying. That’s my experience listening to Nigerian English. Didn’t realise the guy working next to me was speaking English with his compatriot for two weeks and then suddenly I understood. Written Portuguese and Spanish are very similar, like Norwegian and Danish levels of similar, or Scots and English.