[DonRotolo]

OK, to summarize:

1. A 'normal' multimeter is not sensitive enough to measure such low resistances. There are such meters, and one can also calculate resistances using current flow and voltage drop (use 10 A for example and measure the voltage across the wire, you can detect quite small resistances that way).

2. Yes, it is a good goal to keep the highest-current-carrying wires as physically short as possible, there are measurable effects.

2a. And, copper is heavy, another reason for keeping it short.

One thing not mentioned: The connections between the wire and the other components (e.g., breaker, battery, PDB) also need to have low resistance: 0.1 Ohm there is way too high. The higher the current, the better the connections need to be.

0.01 Ohm at 100 A is 1 volt (which you lose to heat), but also this is 100 Watts, which gets real hot real fast.