how to get it

The link to download Mojave via Safari should work if your Mac supports Mojave.

https://support.apple.com/en-us/HT210190

Yes this is an unfortunate new tactic by Apple if you are running macOS Big Sur you cannot download

any older OS's.

Maway31 Said:

"how to get it: can't download Mojave because I have Big Sur installed"

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Downgrading to Mojave:

Which macOS can with your Mac? The one macOS versions that are of option to you are (A) the one that came installed on your Mac, and (B)afterwards (until incompatibly hits again).