For crying out loud…
I hit update on both IOS and MacOS. IOS completed download first (obviously), but then spent a long time "preparing" the update.
Then MacOS completed downloading 3.73 GB and started to "prepare" the same way.
Meanwhile, IOS was done "preparing", and so I hit install. BIG mistake, because then MacOS said the "download failed" since there was no internet connection.
Why do I need the internet connection AFTER the download is complete, and why can't it keep the frigging files cached, since MacOS just loves caching everything?
Redownloading 3.73 GB then… over 4G. Apple is the new Microsoft, the king of stupid software.