how to reindex address book

Always make a backup before trying things like this. An easy way is to back up the Contacts is to use the Export > Contacts archive command in the File menu.

The files are in your user/Library/Application Support/AddressBook folder.

The index files are named addressbook-v22.abcddb. There are now additional files named the same, but with additional extensions. I delete them all. Contacts should rebuild the database from the Metadata folder.

Also, in the Sources folder is subfolders that may contain those files. The Sources folder holds the non-local accounts, like iCloud. Disconnect those accounts before clearing the index files, then reconnect after.

No, that file at the root of AddressBook is for the On My Mac account.

The Sources folder holds the same info for networked accounts like iCloud.

You need to dig into the subfolders of Sources and determine which account that folder applies to.

You can find a string in the Configuration.plist which will indicate the account. It's not the same place in the various account types, so I can't point you to a line or an element name.

However, If you disable Contacts in the iCloud System Preferences, and tell it to remove them from your Mac, it will remove the iCloud folder. Then, recreate it when you re-enable Contacts in iCloud.