It’s quite easy and well-documented:

https://support.apple.com/en-us/HT201372

I thought you had given me a way for something that has troubled me repeatedly. Alas no.

On trying to download Mojave from Catalina I get "Update not found. The requested version of macOS is not available."

The system seems to try and be clever and uses software updater, and software updater says I can't have that version.

So how do you get Mojave?

Edit: On further testing, maybe this is a regional issue? I'm not in the US and the link is to the US app Store. This makes the App Store app unhappy ("Cannot connect to app store"). Changing the URL to my region doesn't help. Same issue for all downloads that predate my current version (Catalina). El Cap and Big Sur download nicely.

> Plug the bootable installer into a Mac that is connected to the internet and compatible with the version of macOS you're installing.

Why does it have to be connected to the Internet? The point of using a USB installer is for it to work offline.

> A bootable installer doesn't download macOS from the internet, but it does require an internet connection to get firmware and other information specific to the Mac model.

The presence of bundled apps which I don't use realy bother me, and always have. On old versions of macOS, I wiped all of them out with a post-installation script. sudo -rm did the trick, and never broke anything else. (They sometimes came back after major updates, but I had a "post-upgrade" script for that.)

Big Sur makes this incredibly painful though, with its root filesystem stuff. Not sure what I'll do if I ever need to use that version. It may or may not be worth it to modify the snapshot...

You can (on most MacBooks at least) restart in recovery mode and do a partition wipe + fresh install with a WiFi connection.

I've bookmarked directions for it. For the admittedly out-of-date High Sierra, for instance: https://www.howtogeek.com/285922/how-to-create-a-bootable-us...