To set the Idle Session Timeout in the admin center, you need to go to the "Org setting" section of the Microsoft 365 admin center, access the Security & privacy tab, and select Idle session timeout.
The "app enforced restrictions" allow idle session timeouts to trigger just on unmanaged devices based on the settings in the admin center or at the application level. This applies to Sharepoint Online and Outlook and needs to be configured for each if you don't have it already set at the admin center level. (Otherwise the admin center settings will trump whatever is configured for Sharepoint or Outlook.)
To configure app enforced restrictions for SharePoint Online, for instance, you need to navigate to the Policies section of the SharePoint Online admin center and open the Unmanaged devices page located under Access Control. In the slide-out menu you can select the level of control (15 minutes, 30 minutes, etc) and click save.
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