Current is electrons moving.
When a circuit is open, electricity can not return to ground.
When a circuit is closed, electrons can flow. The amount flowing is measured in Amperes. This is similar to quantifying liquid or air flow in gallons per minute.
The amount of electicity flowing through a circuit is generally determined by the resistance of the devices, and the voltage the electricity is being supplied at.
When there is a short to ground, there is little to no resistance, and the current flow is going to be very high.
If there is a test light in the path of flow, it will provide the regulation - which is why someone earlier mentioned it as a testing method.
If you are going to use continuity or resistance for trouble shooting, then with the battery disconnected, systematically look for continuity (or low resistance) to ground.
So lets say supply wire to the ignition switch connector has been pinched against some metal.
With the battery hooked up, the flow path would look like this.
View attachment 1715358279
Notice the ammeter reading?
What's happening at the fusible link?
Installing a lamp into the flow path,
View attachment 1715358280
Notice the ammeter reading?
Using an Ohmeter, the same problem is identified like this. Right?
View attachment 1715358304
When you disconnected the wires at the ammeter, you began the process of narrowing down where the ground connection is located.
Make sense?