What Apple operating systems work with iCloud Tabs across devices?
One of the many elements you can sync across devices with iCloud are the tabs in Safari browsers. With Settings > account name > iCloud > Safari checked in iOS/iPadOS, Safari checked in the iCloud preference pane in macOS 10.14 Mojave and earlier, or Safari checked in the iCloud view of the Apple ID preference pane in Catalina or later, your tabs sync, as well as bookmarks and Reading List items. Apple goes so far as to name synced tabs “iCloud Tabs” and synced bookmarks “iCloud Bookmarks.”
In macOS, you can get an expansive view of on-device tabs and a list of those shared via iCloud Tabs.