Marriage Game

Another stupid question from me perhaps.

I played a little more now. Created the Kkngdom of Ireland at around 830 (Start 769) then i went ahead and got the Kingdom of Wales aswell, but it was kind of a chore trying to get county after county. I have Read online to get claims via marriage so your son inherits both kingdoms and whatnot but i seem to be to stupid to comprehend what im reading.

How does this marriage stuff work. How do i get my son to claim another kingdom. What if i have multiple sons? What inheritence law should i aim for before having primogeniture? Anyone got a good yt Video to suggest or can explain it for a Dummy?