How do the microcode upgrades get delivered? Do people have to manually install them, or do Intel have some way to force a microcode update over the Internet?
On Windows it's handled in Windows Update, and maybe there's a way to disable loading the new microcode. On Linux, it's explicitly provided to the kernel by whatever userspace you have. On Arch, for example, it's in a separate package called intel-ucode.
Some board firmware loads updated microcode when it's updated. It has to be loaded at each boot by software in order to change.
On many newer CPUs, the memory for the update is volatile, so it gets rewritten at every power-on. That could be done by your motherboard or the kernel of the OS you are using.
- through BIOS updates (so the motherboard OEM has to release a BIOS update for that model that includes newer microcode and you'd have to download/apply the update)
- through the OS: both Linux and Windows can and do load ucode at boot, depending on the version/configuration/etc
