Help I’m confused!!

Hi guys! I just finished Iron Flame and I loved it! (Which I know seems to be a hot take haha) that being said I’m SO confused on what venin are. Are they just regular humans who got their magic from the ground? Can anyone turn venin? I especially don’t understand what’s going on with Jack Harlowe. The book says he turned before he died? Also is the school keeping him alive as like an experiment? Can anyone help a girl out with this? I’m about to start the third book and want to go into it clearer haha. Thank youuu!!