No brainer here and no secrets. Lots of weight on the barbell, strap it on your back and get after it. No way around it.
Lots of weight on a bar, grab it and pull. No way around it.
You push heavy weight and pull heavy weight, your legs will grow. They have NO choice.
Now with that out of the way, here are some exercises you might want to look into.
RFESS (rear foot elevated split squats) (AKA Bulgarian split squats)
Front squats with a clean grip if possible
Barbell lunges / Barbell stepups
Conv deads / sumo deads