How tf do lice even appear

So earlier last year I cut my hair short like a Bob after a failed wolfcut that my hairdresser gave me and all the life in my hair was gone like fuckass squeaky clean from lice not until it started growing again this July dude it fucking itches as hell at first I was like maybe it's just a few lice but no it was not it was a bunch of baby lice like how TF do they even spawn out of nowhere I'm actually gonna fucking cry because there's too many like I haven't had this shits for years also I do change my pillow case every weak so idk how TF they appear d