Part 121 is NOT Part 121 - in other words, they can, and often are, very different. Let me explain.
FAR 121 is an Air Carrier certification for aircraft that carry over 7,500 lbs of freight. Even under Part 135, you could, in theory, have a 747, provided you limit the "payload" to 7,500 lbs.
Not all 121 Air Carriers are "Flag Carriers" - a Flag Carrier is one that has international authority (thus, the "Flag" designation), and many 121 operators do not.
121 may be cargo only - and, as in the case of Atlas when they added passenger authority to their ops specs, they had to do a lot more work that took 1.5 yrs, and a lot of money, etc.
FAR 121 has a large section dedicated to those 121 carriers who are Supplemental Air Carriers - they have many different specifications - for example, a Supplemental Air Carrier does not fly schedule - only charter flights. The Supplemental carrier does NOT need a dispatcher's authority to dispatch an aircraft - the Captain can do that.
There are many, many different types of operations under FAR 121, and they require many different types of manuals, training programs, dispatch authorities, geographical restrictions (NATRAC-the north atlantic tracks, AMU-areas of magnetic uncertainty, RVSM-flights above FL270 in certain areas, to name a few) -
Certainly 121 Scheduled (as opposed to Supplemental) with Flag (Int'l) authority, has the most hoops to jump through.
So - as I said before - it always looks simple from the outside where you do not understand the inner workings. Don't feel bad, it takes decades to learn the subtle differences and work with them, and from what I have seen, the Baltia team easily has that expertise.