You used to be able to do this when you were 2. If you can do it when you are 20, you are ahead of the game.
This is the kind of mobility a weightlifter works toward. It is the kind of sagittal plane mobility that is part of good joint health and good back health. See how the shin and torso are parallel?
Squatting should be part of your movement vocabulary. Find someone to help you learn how to squat.
Start here and work your way into the frontal and transverse planes. Be patient. Be consistent.