When a battery says it is x amount of volts, what resistance is that based off of?

Battery internal resistance is varied relating to its physical size and chemical structure. Since different type of batteries have their own fixed potential while they are made, like dry cell is 1.5V , NiCD cell is 1.2V, lead acid cell is 2.1V, lithium cell is 3.7V. We common use this standard voltage rate to calculate its internal resistance. Other factor is the current, common use its average AH rate that was marked on its body .

For example: 3.7V cell has products like 1AH; 2AH

then internal resistance can be R1 = 3.7V / 1 = 3.7 ohm

or R2 = 3.7V / 2 = 1.85 ohm

A battery is not normally treated as an "Ohm's Law" device.

The "nominal" voltage rating of a battery is it's "open circuit" voltage; i.e., with no current flowing. Most of the time, you can treat it as a source of constant voltage in your circuit design.

Real batteries do possess an internal resistance, which, in a real circuit, will cause the battery voltage to drop as current draw to the load increases. This becomes more pronounced as the battery discharges.