A good place to start on YouTube is Bernadette Banner’s channel. She is a clothing historian, so there’s a lot of historical and historical recreation stuff, but she also has a few basic repair and tailoring techniques videos. She wrote a mending book that I hear is much more in-depth than her videos (I haven’t read it). www.barnesandnoble.com/w/…/1139915226
Patagonia Wornwear has a lot of repair instructions for outdoor gear (you don’t have to buy their repair materials). wornwear.patagonia.com/repairs
Reddit “visiblemending” and “invisiblemending” are also very good resources.