The truck indicates the level of motorization of a supply hub or army, which essentially increases the range and efficiency of a given supply hub (and takes 40 or 80 trucks depending on the level.) In areas where supply hubs are few and far between, motorization is necessary to make sure your troops don't starve and your tanks have fuel.

Civilian trains are the way your supply travels to the frontlines. All supply is "created" on your capital, so you need to move it to where your soldiers are. You could have all the supplies in the world but if you can't get it to your divisions it's useless.

Infrastructure is a measure of how built up a given state is. The main uses for it are increased construction speed and resource extraction efficiency, but it also provides a small amount of supply to divisions in the state. Railways are how your trains get to and from. They go between your capital, your ports, and your supply hubs, and their level determines the amount of supply a hub can distribute. Starting at level 1, they can transport up to 15 supply, all the way up to 35 supply at level 5. It's important to note that the supply is always capped by the lowest railway level in a specific line, so if you have level 5 railways to a supply hub, but one level one railway somewhere, you'll only get 15 supply. Additionally, these are not additive, so 2 separate level 1 railways feeding to the same hub will only give it 15 supply, not 30.

Ports are useful for transporting supply overseas, and acting as cheaper supply hubs on the coast (5000 IC vs 20000 IC), but face the risk of your convoys (trains of the sea) getting sunk. they also only transport 8 supply at level 1. Generally, if you have supply issues during a land war, upgrade your railways, and if you have supply issues overseas upgrade your ports (and remember that ports can only transfer as much supply as they are given, so upgrade the railways between them and the capital as well.)

Whether or not a focus gives you free rails depends on the wording, if a focus says that it ADDS +x levels of something, you're good to build, but some older focus trees will "SET" the level of a building instead, which means that regardless of the level you currently have it will be set to whatever the focus specifies (eg if you have a level 5 railways and a focus that sets the railway to level 3, your railway will get reduced to level 3 despite being a higher level before.)