Given the nature of your project, this would involve using Google’s OAuth 2.0 service for authentication and authorization. Google provides useful developer guides on this subject on their website. A step-by-step process would be something like this:
– Initiate an OAuth 2.0 flow from your application
– Direct the user to Google’s OAuth 2.0 server to authenticate & authorize your application
– Google’s server redirects back to your application (with an authorization code – single use) after user approval
– Exchange this authorization code for an access token from Google’s server.
This access token grants your application access to user’s GA4 data on their behalf. Note however that this will still require a Client ID and secret from Google Cloud Console for your application. It is significant to note that due to security concerns, direct use of user ids and passwords without such a client key isn’t advisable (or possible, as far as I know).
For accessing GA4 data, you will likely want to utilize the Data API which provides access to report data in Google Analytics 4 properties. This describes all the ways to pull data reports from GA4 properties.
Lastly, do remember that as this involves handling sensitive user data, be sure you’re in compliance with all relevant legal and privacy considerations (including Google’s user data policy when using OAuth 2.0). It would be advisable to seek legal counsel if you have concerns about handling this data in a legally compliant way. Good luck with your project!