Hello maester Steven, i was wondering how did robb manage to remove sansa as his heir by just writing it in his testament? From what i have read it seems to be pretty difficult, neither robert nor tywin were able to disinherit joffrey or tyrion even at the height of their power. Thanks for your answer!

Robert could have, but decided not to, although Eddard basically did it for him by altering his will; Tywin was waiting for Jaime to be removed from the Kingsguard, but fully intended to disinherit Tyrion.

As for how Robb did it, he basically wrote it down in his will that Jon was his heir specifically, ahead of all other claimants.