The following are three key and core popular topics of SharePoint Online:
Project management - SharePoint Online can be utilized to manage projects via tools for tracking tasks, deadlines, and progress. This can be done via using a simple out of the box list or custom list or integrating via MS Planner.
Social networking - SharePoint Online includes social networking features such as Microsoft Viva Connections. It also includes the ability to utilize via the out of the box people search integration with employees Delve profiles to have better employee communication and engagement.
Custom development - SharePoint Online can be utilized to develop custom applications and solutions. One can use SharePoint Online to extend the functionality to meet the specific needs of the organization. To do this the use of JSON (JavaScript Object Notation) or SPX (SharePoint Framework) maybe needed.