Doing this for MacOS is surprising hard. Fortunately crapware isn’t a thing, although a few of the built in apps seem to try and drive me demented (looking at you Siri, Stock, Music, auto update).
Why does it have to be connected to the Internet? The point of using a USB installer is for it to work offline.
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.
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...