Disk rot usually happens when air gets in contact with the reflective coating and oxidises it. With CD’s, it’s actually the top side you need to be worried about, as it’s right there under a thin lacquer coating. Any ding to that can expose the layer or just literally chip off a chunk of data.
At least on DVD’s it’s sandwiched inside the disk, so usually the only reason is a manufacturing error, and not really something the user can cause.
Sideloading (is supposed to) mean just transferring files between local devices, as in, you aren’t uploading or downloading them, you are going sideways. E.g you download a song to your pc, then sideload it to your mp3 player.
That’s also where it comes to installing phone apps, as you transfer the app file to the phone yourself. Being “outside” or “going around” the official method just happens to be another way to interpret the meaning of “side” in the word.
Also it accelerates the design-to-manufacture cycle of a new model - just slap a huge touch screen on it and start building the car, and hope the software is ready in time. If not, well, just ship it as is and patch it later.
That would require starting a database with the purpose of cataloguing every single part number in every single device that exists, which while technically possible, is rather unfeasible without extensive manufacturer cooperation.
What iFixit is doing is the other way around, they are telling what device a certain part number they carry fits in - as in their example, what Lenovo laptop that specific battery is compatible with. That’s a problem multiple orders of magnitude smaller in scope.
In a perfect world though that information would be available in the repair manual and schematic that came with your device, as they usually did a few decades back. Alas, that’s something that’s never going to happen again because it hurts profit margins.