Sure of which part? I try to add qualifiers like 'I believe' or "I understand' for things I'm not so sure of but I still make mistakes about certain things as this game has a lot of things. Confused historical rivalry with 100% chance to mutually rival in 1444 with regard to France and England recently, for example
What I believe to be the case is this:
When you release a vassal: they get ideas from a predetermined list and as such if you release certain nations you can get colonizers for free. It may be that they combine the ideas that had previously had while still alive with ideas from their list. I'm not 100% on the details. I have definitely released Castile after integrating or possibly outright conquering them in order to get them to colonize for me.
When you vassalize or PU a nation that does not have Explo/Expansion: they will not pick either of those groups. They still use their normal weights to choose ideas but those weights include ones to prevent subjects picking Explo/Expansion.
As an aside, in an observer game I ran recently, I saw Aragon take both Expansion and Exploration and colonize a huge chunk of Brazil. Never seen Aragon colonize before.