How to subscribe to calendars so they are synced to all devices?

If I subscribe to a calendar in the iOS calendar app, it is only visible on that device: However, in the past I’ve been able to add a calendar subscription to my iCloud account so it syncs across all devices: I can’t see for the life of me how I did this. I think it was from the settings app but I can’t find the option on my iPhone (iOS 18) or iPad (iPadOS 17). I’ve read I can do it from a Mac, but I don’t own a Mac so I know I never did it that way. How can I add a subscription so it’s synced to all my devices?