This is a nice post from developer Simon B. Støvring about getting data out of 1Password and into iCloud Keychain:
Ensuring all items in 1Password have a valid website address is necessary in order to import the items. iCloud Keychain will skip any items that do not have a valid website address.
After ensuring all items have a valid website address, they can be exported from 1Password by selecting a single vault and navigating to File -> Export -> All items…. After entering the Master Password the dialog below is presented. It’s important to change the file format to “iCloud Keychain (.csv)” before exporting.
The article also includes a link to Ricky Mondello’s shortcut for opening the Passwords interface directly on macOS or iOS.
[Once again, a note to make it easier to find.]
Source: Six Colors