It gets in because the air they pressurize the cabin with comes from the engines.
I don’t recall exactly where or how, but obviously ahead of where the fuel is normally injected.
I am sure an A&P Mechanic will be along shortly to clarify. (And make me look stupid..LOL)
You are correct, and I am not a AP mechanic. But if you look in google, most pressurization systems bleed air out of the compressor stage in the engine, cool it, then inject it into the cabin. The engines don’t even notice it. Well, maybe some software somewhere that controls the fuel injection system notices....
I used to be a jet mechanic in a former life, and you did fine explaining it. I understand why that would confuse the poster...it isn’t something that is apparent.