Customer overview contains a list of your active customers. You can use Account owner to filter customers. This way you can quickly find active customers for a specific person or persons.
When you need to report Customer data more specifically, go to Reports > Report creation. In report creations view you can create your own customer based reports and save them for future if needed.
On the top of Overview page you'll also see relevant KPIs related to the content. Check out the formulas used in KPI calculation.
From Customer overview it is easy to open a specific customer card or add new ones if needed.