Is it actually necessary to use a paid authenticator app? Because why not you use Google authenticator app, which could be found on either Play Store and Apple Store for your Two factor verifications.
There are many Two Factor Authentication Applications and you can have many free ones to use, including open source 2FAs.
A good open source 2FA is Aegis.
https://getaegis.app/You can use Google Aunthenticator but it is not open source and you can not expect open source products from Google.