It should be noted that infrastructure below a ''base'' level of two has no throughput, but if any infra starting from a 2+ base is damaged (such as by bombing) and goes below two it will still throughput supplies.
We don't know exactly how the supply system determines what supply paths to create, nor do we know how it prioritizes them, but we do know how they have to function. For illustrative purposes infrastructure can be viewed as a multilane highway that only allows a fixed number of cars through per day. So, if you have a province with 10-lanes of infra with a throughput of 800-cars, feeding three other 10-lane provinces with the same throughput, the maximum combined cars those three provinces will receive is 800/day (even though they are capable of passing 2400-cars). If at any time the three provinces demand more cars than the first province can provide, that province will raise a red flag in the form of red hatchmarks and the supply system begins to break down. This is the "supply bottleneck" that leaves many armies starving on the Steppes of Russia.
=== Ports and Supply Convoys ===