A load has a response curve - for each value of applied voltage it takes a particular current. This
response is a property of the load only.
If several loads are supplied from the same (voltage) supply they act independently as each has
its own response, the presence/absence of other loads is not visible (assuming the supply can
maintain the voltage properly).
Thus as you add loads to the same supply the currents add up (until the supply becomes overloaded,
of course).
Motors are more complicated loads as they vary with time due to mechanical loads.
If you want to drive motors, which are inductive, you need to know about preventing inductive
kick-back (typically using free-wheel diodes).