What Happens When an App is Processing

After running into some issues with how our code runs now compared to how it did 6 months ago (with the same exact code base) leads us to wonder what happens when our app is uploaded and processed for the app store. What is the process? Is our app compiled on Apple servers? How often does Apple change or update their tools/software? Is there a change log available? I'm having trouble finding documentation or much mention of this. Any help or wisdom would be appreciated.

After running into some issues with how our code runs now compared to how it did 6 months ago

Could you expand on this ?

The only thing I know about it is that they trim the libraries to go from fat to smaller apps, be keeping only what's needed for each type of device.
What Happens When an App is Processing
 
 
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