...
- Google prompts - (Google default and it happens on any "eligible phone" you are signed into your Google Account)
- Could be multiple devices
- Voice or text message (multiple phone types - can't use Google voice)
- Backup codes - (requires a printed copy of codes)
- Authenticator app - (requires smart phone)
- Security key - (requires purchasing a key - yubikey is the most famous - https://www.yubico.com/products/ - also available on Amazon)