What is going on during a macOS/iOS update?

Updating a Linux based OS is just a matter of downloading a few packages. If a kernel update is involved, you just restart afterwards. All done within a minute or two. Similarly with Windows, it takes a bit longer and the update is finalized during a restart cycle but again it's finished within a few minutes.

On the other hand, updates on macOS (and iOS) take several gigabytes to download (Security Update 2020-002 for Mojave is over 1.6GB, Catalina 10.15.4 update is over 3.3GB) and take 10+ minutes to install. Not only that, the machine is restarted several times during the update, the screen often goes black, the progress bar is all over the place, sometimes it looks like the machine dies with fans being instantly cut off just to wake up again.

Is there anybody who has any insight into what is going on? Why are the updates so freaking large? Do they essentially replace entire parts of the OS instead of updating? Why does it take so long to install them given how fast the SSDs and CPUs are? And why is the machine restarted several times? Are the updates applied in stages?