It's way too complicated, but from what I can gather, there's a couple different accused issues.
There's inflated revenues, which, arguably, because the entity that owns City, likely has some stake in the places that are doing the overpaying, or at minimum, companies who want to do business with the entities that own the club are overpaying for gain down the road.
But there's also a bit of supposed off books payments to players, agents, associated projects and the like.
The first issue is much harder to regulate and prove. Everyone knows its phony, but that might be harder to dispute in court. The second issue is also difficult, as unless you have a smoking gun of proof, you can't fully prove it either. There's a lot of grey area in contracts, agent money, and other financial payments that count for FFP, and some that aren't really clear.
It's clear that City likely was involved in both and probably to a massive extent. But how you prove it and how you punish them is complicated unless you have cooperation. What's the incentive to cooperate for parties involved?
I think it's more likely you see some type of retroactive punishment, a hefty fine, and a 'nod nod, wink wink' to operate more in the open in the future. At this point, City's revenues, player accumulation and winnings have likely accomplished what they had set out, no matter the punishment. What likely was inflated sponsor fees for things could be closer to reality because they have won everything they should.