Make sure that all of the devices that you want to be included in sharing are using the same iCloud ID.
Then on the devices, under Settings/Preferences>iCloud, you will see a list of shareable items; check Calendar if it is not already selected.
On a Mac since 1984.
Currently: 24" M1 iMac, M2 Pro Mac mini with 27" BenQ monitor, M2 Macbook Air, MacOS 14.x; iPhones, iPods (yes, still) and iPads.
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