Repeating this "Instagram is listening" conspiracy theory does a disservice to many legitimate privacy issues.
Alexa listening was also a conspiracy theory until it was discovered it did just that and Amazon claimed it was a “rare occurrence.”
The incentives speak for themselves.
If Instagram was using private apis that would allow to hear your mic (and I don’t even think they exist), what’s the incentive for apple to cover it up? They have been attacking Facebook/Meta relentlessly when it comes to privacy.
It’s just not a thing. Burden of proof would be on your side to show how this can be done.