Idon't understand why when you use a straw the higher pressure outside of it pushes the water up the straw.

Pressure is force acting on an area .
in the same area, more pressure means more force.

2 person, pressure outside and one in straw push water against each other. more pressure outside, so more force it pushes ... where will the water flow?

Velocity is the speed of an object in a certain direction so you'd need to know speed and direction