on our hand is the knuckle the fulcrum? and where is the input and output force then?

Answer 1

For a finger:
The knuckle is the fulcrum
The load is applied usually at the fingertips in pressing something
The effort is exerted by the muscle linking the upper part to the lower part of the finger or to the hand.
This makes the finger a 3rd order lever:(F;E;L)