In your database, the Member Tables relate to each other in the following way (Not all tables are shown in the image below because of the amount of tables and relations in it):
In the image above which is an abstract part of all the relations of the Members tables, we can see that the main table that holds users information is the users_data table. This table by default holds more than 25 different types of information for each member. Based on the ID each member gets on this table, it extends its relation with other tables that hold additional information. For example the data_posts and users_portfolio tables hold Posts from the different Membership Features members have access to. The users_favorite and users_billing offer members the information the name of the table suggests (Favorites and Billing). We can even see that, for members to appear in top categories and sub categories (Profession and Services), they are related to the list_professions and list_services tables through the rel_services table which links the IDs of the profession and services to the IDs of the members. Lastly we can even the users_meta table which holds custom information that members have (eg: Custom Variables found in posts, forms and any type of input that adds a new field that is not found by default on the site).
So if you wanted to search for all members that have a photo, you would use the users_data and users_photo tables. If you wanted to search for all members that have any posts created you would us the users_data, data_posts and users_portfolio. And if you wanted to search for members with a specific profession you would need to use the users_data, rel_services, list_professions and list_services.