[Jasper_]

A majority of microcode updates aren't actual code, but are programming large swaths of undocumented registers known as "chicken bits" which turn off functionality and affect system operation. You can see what a rough fix looks like in Linux patches for devices that don't get these microcode updates: https://lore.kernel.org/lkml/b4ad7273efbb0c60a6c93ae68f82a44...

No, the details of what these MSR registers do isn't public. But it's far from being code; it's simply rather tweaking a large switchboard of functionality which already existed on your CPU. It is not adding new features or code to the execution pipeline.

Modern process development pipelines are already far too long that a common thing to do is to put an experimental feature in all CPUs and only enable it with the right microcode and chicken bits when it works well enough for general use. It's not uncommon for new processor features to have a 5-6 lag from "first buggy implementation" to "general implementation".