Google Authenticator

It will be sent to any and all devices signed in to the relevant Apple ID that are trusted.

iOS devices automatically become trusted when you sign in to the iCloud account in settings.

Computers and browsers will have a Trust prompt when signing in.

You should not share iCloud accounts or Apple IDs at all. An Apple ID identifies a single individual. Apple does not support the use of an account by more than one person.

You can share an iCloud Calendar with other people without needing to share an iCloud account.

click here ➡️ Share a calendar on iCloud.com - Apple Support

You cannot make a non-Apple device a trusted device. By definition, trusted devices are your Apple devices signed into iCloud with your AppleID. AppleID 2FA codes are sent by default as encrypted iCloud notifications. They are only ever sent as SMS text or automated voice message when you make a request to receive a code to your backup telephone number(s).

Codes are also only useful for about 10 minutes, after which a new code will be required.

As a company, you should not even be allowing employees to use personal AppleIDs on company devices. Apple’s solution for company deployments is to use a managed deployment or MDM, NOT personal AppleIDs.

Business - Apple

You cannot. Apple's 2 Facor authentication does not use or require a third party app.

The code is sent to any trusted device that is signed in to the Apple ID.