The PARC machine barely scratched the surface of what could be done, and it was far from being packaged in a way that could be marketed to consumers. It was inspiration, and any engineer knows that's a long, long way from a working product.
Usually they've caved in to the conventions of the market (iOS notifications) or missing features (F.lux). The amount of blatant theft seems minimal. They usually put their own spin on things to differentiate somehow.
Even Microsoft has gone its own way with Windows after some heavy overlap with MacOS and other windowing systems (DESQview in partcular). Windows 3.1 was more different, and Windows 95 borrowed more heavily from OS/2, which was wildly different from MacOS. If anything Apple was forced to adopt the right-click menu popularized by Microsoft since it became the de-facto standard.
Meanwhile other companies have damning memos that say "Make it look exactly like Apple's product".
Sweeping generalizations like this aren't helpful when the degree to which these things occur is extremely important.
Xiaomi has built its entire company on blatant theft of designs. Let's not say "oh, it's okay because Apple once did X" which is like saying murder is no different from a speeding ticket since both things are illegal.
They were never first to market with a category of product, that's just not in their DNA. Even the Apple I was a computer largely similar in terms of features to a multitude of others on the market, but it wasn't a direct copy of any of their competitors.
Their ideal product entry point is when consumers are familiar with the product but are unhappy with the current offerings because they lack the right contextual fit.
Maybe if you have a specific example here of where Apple's flagrantly copied something like Xiaomi has we can talk specifics.
Apple Maps, Apple Watch, F.lux, Those are just off the top of my head, Apples been around a long time and its happen many times (but as Ive said all companies to this). I just don't like deniers where Apple is viewed as all good and in the light, when they absolutely are not. (and neither are most companies) Apple is just made because its happening to them right now and not the other way around, its eating their potential revenue.
Apple Maps: In what respect is this "stolen"? By the same argument did Google steal from Garmin or MapArt who in turn stole from cartographers? Not sure what you're saying here. Mapping applications have been around for decades. Apple needed one on their phone. They made one. It's similar to other mapping apps, Google's included, but also took a new approach in some regards.
Apple Watch: It's a smart watch, they didn't invent the category, but it's a novel form-factor, instantly recognizable. The operating system is also highly distinct from others. You'd never confuse an Apple Watch for anything but an Apple Watch. Some of the Android based ones are going out of their way to look like non-smart watches. At least Motorola has their own design language here.
F.lux: Significant functional overlap but these sorts of tools have been around since the 1990s. It's not like F.lux invented this, they just popularized it. Maybe it was a dick move on Apple's part, or maybe it was just a thing they thought should be core to their product.
When Microsoft wanted to add faxing functionality to Windows they could have bought WinFax, but they instead made their own thing. Is this stealing?
I think your definition of stealing and copying is extremely loose, to the point where the term has lost all meaning.
Your comments use words similar to mine like "the" and "a". Are you stealing my words?
Apple created Apple Maps and at the same time did not approve Google Maps for iOS6 (they obviously later approved it because the uproar was deafening). Like I said those were only the first few things that came to mind. Apple steals ideas, Google steals ideas, Microsoft steals ideas, Facebook steals idea... They all do it, that was my only point. And yes I stole your words, please pay me for them.
This would be analogous if the Mac was named the Apple Alto and looked like almost exactly like the Xerox Alto and even cloned the marketing materials and presentation templates that Xerox used, if Xerox had ever made the effort to come up with distinctive marketing for their computers (which they didn't, but that's the bar for "this situation is like that situation").
http://web.stanford.edu/dept/SUL/sites/mac/parc.html