How do current and voltage relate to load?
Every different type of circuit element (resistor, voltage source, diode, capacitor, etc) has a different constitutive relationship --- the relationship between voltage and current (possibly time-dependent) for that type of element. 

For a resistor the relation is \$V=IR\$, but for a capacitor it's \$\frac{dV}{dt}=I/C\$. For a transistor there's a different equation, and so on. 

To solve a circuit, you generally have to combine these relationships for the particular type of devices used in the circuit, with the general rules given by Kirchhoff's laws, to form equations that (hopefully) have a unique solution, which tells you the operating point (or time evolution) of the circuit voltages and currents.

For something like your phototransistor, the relationship is something like

$$I = I_0(P_{opt}) + V_{ce}/R_o$$

where \$I\$ is current out of the base, \$I_0(P_{opt})\$ is the incident-light-dependent photocurrent, and \$R_o\$ is the effective output resistance of the device. In some cases you might be able to ignore the output resistance term, but in others you might not get solvable circuit equations without including it.