What in your opinion makes, say a Dell OptiPlex 9020 a Dell OptiPlex 9020? Is it the lackluster case? The plain green motherboard? Something else? Thank you for your time!
What makes a Dell OptiPlex 9020? The manufacturer -- Dell in this case -- has an internal document that describes it. It's called the Bill Of Materials (BOM). The items on that list combined make up an OptiPlex 9020 according to the manufacturer.
It's also important to understand that same BOM is submitted to various government agencies around the world stating it to be so. In some cases a physical sample must be submitted (especially to telecommunications and electronics regulation agencies). There are also tariff and tax implications due to what is on the BOM.
Note that the model number is mostly a marketing concept. You can have different BOMs but they can both be marketed under the same model number. Samsung has done this with some of the Galaxy smartphones, putting different SoCs based on the target market. Apple has released different part numbers for the same iPhone model due to cellular radio frequency coverage differences.
And of course a manufacturer can state that certain parts could be used (like capacitors).
From a consumer standpoint, it's basically what rolled off the manufacturing line and given a serial number after passing QA. This isn't just a PC thing, it's the same for a toaster oven or even products that don't have serial numbers (like a pair of OXO Good Grips 10" kitchen tongs).
So even if similar internal components to an OptiPlex 9020 are stuck in an XPS chassis, it's not an OptiPlex 9020.
From a PC nerd standpoint, it's really a combination of the case plus the main internal components: motherboard, CPU, RAM, GPU, storage, power supply, et cetera.
Note that the operating system can also have alternatives. I know Dell has offered some of its models with the buyer's choice of Microsoft Windows or Linux.
Oh, if you stick a Gaggia espresso machine in a purple computer chassis
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it's not an SGI IRIS Indigo workstation.
For sure it's not the case. If I stick a Raspberry Pi 4 in a Mac mini 2018 chassis, it's not a Mac mini 2018 because that's not what was on Apple's BOM.