iOS is for iPhones and iPads.
macs run on Mac OS X or macOS.
Installing a newer OS is called an upgrade.
Which OS you can upgrade to will be governed by which model and year your mac is.
Click on the top left of your screen click on About This Mac to find out the year of your mac.
For macOS Big Sur,
Open Safari and click on this link,
How to get older versions of macOS
Go to Check Compatibility to see if your mac qualifies for macOS Big Sur.
Then go to the Use Safari to download macOS section and click on macOS Big Sur.
Before upgrading make a full back up of your Mac.
Check that third party apps you use have updates available for the new OS.
https://roaringapps.com
Check that your printers/ scanners etc have updated drivers and software available for the new OS.
Connect your mac to your router via cable rather than rely on WiFi.
Do not use your mac while it is downloading
and do not use any other devices that share the same internet connection.
If the App Store says ‘Open’ then it is detecting a previous download of the install app
go to the Applications folder locate Install macOS ******.app.
Move it to the Trash, empty the Trash.
Restart, click on the link again, the App Store should say ‘Get' or 'Download'.
If you want to or are able to upgrade to Monterey open Safari and click on the link below,
How to upgrade to macOS Monterey – Apple Support (UK)