And yes, I worked in AdTech. People really overestimate how many people need to know these things, and how likely people are to whistleblow when the "crimes" are morally grey.
If there's a giant friggin' third-party endpoint feeding your engine a firehose of data that the firm's whole business relies on, everyone will know about it.
These systems are held together with spit and bailing wire, everything breaks all the time, everything's idiosyncrasies have to be worked around all the time, and all of the component behaviors have to be well-documented for integration and testing and release and design purposes.
This might fly in a small shop, not so much in one with thousands and tens of thousands of eyes involved. The bigger you get, the harder it is to keep a secret.
But they won't know where that third-party's data came from.
But there are very solid reasons that isn't what they would do (see "It's impossible to keep a secret of that scale in a 100,000 person firm").