Hearts of Iron IV

[Veho Nex]

So supply works on 3 points as far as I can tell.

First you have your home port/supply center. It then follows the path of highest infrastructure via land or from highest port to highest port via sea. Lastly you have the infrastructure of a province that prevents the army from taking attrition. I don't know exact numbers for what each level does but that's how I've seen it behave.

Incorrect:

Infrastructure determines how many units can sit in one province as far as I can tell. If you have something in Siberia with infrastructure 10 surrounded by a bunch of 1s it should still be able to sustain a pretty large army. If that army gets weakened it takes longer to return to full strength due to low supply incoming to army.

I noticed the above during an invasion of Japan from one of the islands the USA controls to its north. I only had a port of 3 with infrastructure of 10. I was able to sustain 4 large army groups that were ready to land and take Japan.

Edit: so after doing a little research I found that ports offer 3x their value in supply. So my port of 3 was actually supplying like it was 9. The real answer to that is:

Incoming supply is equal to the bottleneck (lowest value) on the best path from the capital including both the end supply area and the capital. Supply areas are connected with supply values as follows:

By land: Supply areas with a land adjacency. The value is the sum of:
0.5 times the square of the average infrastructure in the sending supply area. Capitals do not receive this factor.
2 times the square of the average infrastructure in the receiving supply area. Note that capitals get this value as "local" supply.

By sea: Supply areas with a sea connection. The supply value is 3 times the total naval base levels in the sending and receiving supply areas, whichever is lower.

Tank, equipment, supplies, mechanized, and motorized go either in surplus (which is where it goes if all units that use it are full of that item) or to units in production or to units in the field.

So when calculating your attack plan accordingly. It may be beneficial to hold off your war start till you have a sufficient stockpile of arms and armor.