Devil May Cry

How does everyone get so much red orbs?

For example, people who were just on mission 4-6 already have air raid, stinger lvl2, and even air hike.(alright maybe not all of them but they still have a lot of skills.)

Although I seem to not have as much money around the time and I haven't spent anything at all on extras.(devil stars, holy water, etc.)

Well, if it's a fresh game, they would have got them by getting S Ranks/Special Bonuses. (An SB is a reward of 2000 red orbs for a "perfect" S Rank - that is to say, obtaining the maximum possible number of ranking points.) If you get SBs on every mission, you can buy every Alastor skill by about Mission 5.

If you're just getting Cs and Ds, by comparison, you will hardly get any red orbs at the end of a mission. If you don't care about rank, then your best bet is to orb-farm hallway Phantom in either Mission 4 or 7. Defeat him, collect 500 red orbs, use Select (or Start, always forget which one it is) to bring up the options screen, save, reload the mission - rinse and repeat.

As a quick tip, hallway Phantom melts before Vortex like butter.