MR CORVENTIA

- Morpho Fat and Skin Folds PDF - holy grail of drawing fat people and old people better

- Fatphotoref.com - reference bank of unsexualised fat models. you need to sign up to be able to access it, but it isn't very hard to do.

- the adipositivity project - photography project capturing diverse bodies

- How to draw overweight characters video - focuses on the fat pads of the face

-The Most Overlooked Life Drawing Topic : Fat

I recommend also studying from artists who depict diverse bodies in their work, as a guide to stylise those bodies and make them work in your own style. As with everything else, studying from reference material is the best way to learn. For more reference material, I recommend looking up plus-size/inclusive brands, and using their model photos as references.