Hello EcoAxis,
Thank you for posting your question in the Microsoft Q&A Community.
I understand you are being prompted to register for authentication at every sign in.
Please be informed that security defaults in tenants control MFA without Premium licenses. The MFA control can be shifted to conditional access once you add a premium license to your tenant.
However, MFA registration can be prompted based on the settings within the tenant. One particular place to check is the Self-Service password reset blade. If you have enabled this, users will be forced to complete MFA registration if they haven't done so.
Also, check within the Entra ID Protection if the MFA registration is enforced
Let me know if this helps.
Babafemi