Somewhat related, that's how guns are classified in the US: The Gun is "just" the lower receiver. Even though that one is useless by itself, it is the foundational piece of it to which one attaches all the accessories. (And even then there's loopholes, like how a 80% lower receiver is just a piece of metal and not a gun yet)
With a car, I guess one answer would be "The part that has the VIN on it is the car", but apparently there's no universally agreed on location. I would probably go for the drive shaft.
Yeah, it just kinda ruins the aesthetic of an all-in-one wedge if you need to connect an external drive, also I'd prefer it to not be USB (I assume you use an NVMe-to-USB chassis?). So I'd love to see that being done internally. Essentially a RPi 5 with an actual NVMe slot and full size HDMI, in a wedge chassis with a better keyboard.
I am hoping that there would be a real Desktop Raspberry Pi in the future. I know that the Raspberry Pi 400 exists, which was clearly aimed at that, but an SD Card as primary storage just ain't it. The Pi 5 supports NVMe, but needs an adapter.
I would like something that has NVMe and non-USB connected networking, with sufficient cooling, with a good keyboard. GPIO should still be exposed (like on the Pi 400). I don't expect a PCIe expansion slot, basically I just want a beefed up Pi 400 to be a true BBC Micro successor. (And for the love of all that's good and holy, proper HDMI ports. And a proper keyboard, not a laptop keyboard like on the Pi 400. Heck, steal the Amiga 600 design.)
With a car, I guess one answer would be "The part that has the VIN on it is the car", but apparently there's no universally agreed on location. I would probably go for the drive shaft.