Asalamu Alaykum Ustadh, I woke up this morning and I felt my underwear was a bit wet (I'm a boy). Do we apply the Qaidah you taught us "Al Yaqeen La Yazulu Bisak" and ignore this (not thinking its urine for example)?

Here several factors play into the ruling, if u had an intimate dream then make ghusl, if u had thoughts before sleep, during, or after that stimulated you, then it is mathi, so you sprinkle water on it and you wash your private area and you do wudhu.