The portal users will get the reCAPTCHA V2 or V3 as you have configured in the reCAPTCHA settings from “Portal General Settings”.
If you have enabled and configured the reCAPTCHA V2, the portal users will get the “I am not a robot” checkbox on the Login page as Google provides. The portal users need to be checked for the form submission.
If you have enabled and configured the reCAPTCHA V3, the portal users can see a captcha at the right corner of the screen. It will send the token automatically based on the portal user’s actions.