The airforce is a component of the UNSCDF alongside the army.
It's a bit of a shame tho, as you say the lore mostly focuses on naval forces.
It's probably fairly similar to the differences between a US Marine and a US Army soldier, since that's mostly what they were based off of. Marines are likely better trained, have stricter requirements and regulations, and have better/newer equipment than Army troopers. It also seems like marines did most of the fighting, since the Army has only been seen (or even mentioned, really) on Reach.The Army did its fair share of fighting (ex; Algolis in Halo Legends where an Army Corps of Engineers battalion was involved), its just that the games and books tend to focus on naval/Spartan engagements...and what do UNSC ships carry for ground campaigns?
They had their fights, we just never saw them in the narrow specturm of the universe we've seen.
Other than that I agree. Though the Army seems to use older stuff that they know and trust that works. Its most likely personal preference more than anything else.
Does the UNSC/UNSCDF even have a air force branch or is it incorporated into the Marine Corps and Army?
Also on the topic of equipment id assume there is a large disparity between training and equipment of a colony like Reach's Army and that of a smaller outer colony.
While the navy seems to have a standardised level of training and equipment.
Also I have to assume the Army is much larger than the Marine Corp.
Correct. In fact the Every Air Craft flown in Halo: Reach was by the UNSC Air Force. From the Falcon to the Sabers.
Also the Army was displayed a lot since 343I took over.
Spartan Thorn's Parents were army officers
the Soldier who would talk to the ODSTs in Halo Legends "Babysitter" was in the Army, so the Army had regiments along side the Marines in large MEUs reinforcing colony worlds.
All Colonial Militia fell under Army command, the only one under Marine command was the one trained on Harvest by Johnson.
There are tons more examples, but it really feels like 343I has been really fleshing out the UNSC Army with its extentions into the Expanded Universe.