Content search in Microsoft Purview allows you to search for specific content across user mailboxes, SharePoint sites, and OneDrive locations. In this case, you want to identify email messages that contain the word Project1 in the subject line. A content search will allow you to specify the keyword "Project1" and narrow down the search to the mailboxes of specific users who worked on the project.
User data search is not a feature in Microsoft Purview that matches this requirement.
Records management disposition deals with managing records and their lifecycle (such as retention and deletion), but it is not related to searching email messages.
Audit search allows you to search the audit logs for activities performed by users, but it does not search the content of emails or documents.