If I understand right, nothing uses physics unless you add a node with physics.
'_process' is the regular method used in every game engine where you can touch things before every draw, delta (from delta time) is the time lapse you can use as integrator to avoid jitter.
'fixedprocess' executes every physics frame, a slower and almost fixed delta time, where is advised to manipulate physic objects, again, delta as your friend when moving things.
So, neither process has physics, your jitter could be related to something else (like rounding of some variable).