In drum brakes i think there is a spring that returns the shoes of the drum to their original location which pushes fluid back into reservoir. However for example in the video at timestamp https://youtu.be/FeuZJ76J12I?t=77, i dont see any return springs used in disc brakes. How exactly does the fluid return back to reservoir after brake pedal is released in disc brakes?
In the caliper, the seal is designed so when you press the brake pedal and fluid enters, the seal twists sideways. When you release the brake pedal, the force of the seal pushes the piston and thus the fluid back, which returns it into the master cylinder. There is enough slide in the seal it allows the piston to move forward if needed to take up space from worn brake pads or during brake maintenance. This action retracts the piston so the pad doesn't constantly stay in contact with the rotor surface.