I don't understand the ammo situation. I never have enough to destroy each car and end up losing because I'm not doing enough. I thought the marks on the ground that aren't the arrows were supposed to give me more ammo, but they don't seem to do anything even when I hear the sound to acknowledge going over them.

Do I need to switch weapons or something? Are there even different weapons? if so, what key on the keyboard does this?

ADG has a complete analysis video and accompanying PDF which documents the actual gameplay differences between the different MegaRace cars. https://youtu.be/5JCVZ4pD3c8

The bottom line is that all the unlocked cars are at a disadvantage. Drive the Ouzbel which is the purple one you have unlocked from the start.