what happens to PCM when it is processed by DTS or Dolby?

Decoded is not the right work.  "PCM" does not decode to a surround format.  It's more like "translated".  It's artificially creating a surround type music experience from 2-channel audio.  It's like "PCM" is like a picture of a two trees in front of you.  Then you as an artist need to imagine what it looks like in the middle of a forest.  That's what these DTS Neural and Dolby Music surround modes try to do.

Netflix can be PCM 2-channel or 5.1 Dolby Digital depending on the show.

HBO Max and Disney are not Dolby Atmos.  Most likely, they are all just Dolby Digital.

Anything coming through cable or satellite is usually Dolby Digital.

That's like asking if a buggy whip  affects the speed of a car.

I have a NAD processor and display is very good.  Shows incoming signal and it says PCM all the time.  Why doesnt it say dolby digital?  
It does say dolby atmos for hbo max and disney.  Thought hbo max says it will play highest quality audio and video avail for your device (using an apple tv 4k box connected to processor).

Sounds like either a configuration in your audio settings for Netflix on Apple TV or it is a limitation of the app.