Managing access to multiple reports for multiple projects in one workspace

The best way is Row Level Security - RLS. As each user logs in, the report filters itself to only show what the Power BI dev allows for that user/user group. You have to write one report.

You can publish a "golden dataset" in one workspace, then create Live Connected reports to it in other workspaces that you restrict permissions to by user/user group. Downside is if you need to make a change to the report, you have to make the same change to all copies (adding a new visual for example)

You cannot restrict reports in a single workspace to a set of users. Everyone with access to the workspace/app can see all reports in the workspace/app.

You may google on ways to hide pages based on user. Do not rely on these tricks. They are clever, but not secure. Though a user may not be able to navigate to a page within the report, if Report User A has visibilty to page 2, and copies that URL to an email or chat and asks Report User B about it, even though User B shouldn't see it, that URL will work for them because they have access to the report.