- How come only some Saints are canonized?
Did they go straight to Heaven or are they just like role models for the Catholic Church?
Only some saints are canonized because
- first, they have to be identified
- second, their virtue has to be witnessed; and
- third, someone has to go through the effort of advocating a cause for them (i.e. promoting their canonization).
Generally the only people with the momentum for this are founders of religious orders. Your grandmother may be a saint, but until someone realizes "Hey, we should canonize her.", it isn't going to get very far.
- Then you have to meticulously document (or have documentation of) their "heroic virtue".
- Then you need someone to push the cause through the bureaucracy and have enough interested people to form a cult (cult in the technical sense of devoted following, not in the sense of an aberrant religious organization).
In the Western Church canonized Saints are generally regarded, first, as role models. Many theologians would say that they go straight to Heaven, but I'm not convinced myself that this is necessary, as long as they are in Heaven before they are canonized. (But then you get into the whole thorny time-in-purgatory versus time-on-earth issue.)
- Is it rare for souls in God's friendship to die and go straight to Heaven, while most go to Purgatory?
Most theologians and Fathers do say that it is rare for souls who die in God's friendship to go straight to Heaven without purification.
- What happened to the righteous souls that were born before Christ after they died?
(like Moses, Noah, Abraham etc.)
I've heard of "Limbo of the Fathers" or "the Bosom of Abraham" but I'm not sure what they mean.
Yes, all these went to the Limbo of the Fathers, called She'ol in the Old Testament (and Hades in Greek mythology) and illustrated in Jesus's parable of the rich man and Lazarus (Luke 16:19-31).
It was just a holding place where all the dead went to await Christ.
- The Bosom of Abraham is where the just went, to be with Abraham.
- The Limbo of the Fathers is the wider term for the place of the dead.
There is not much more to it than that.