Where does the data in my Timeline populate from?

At present, all elements you can see in the Timeline are being sourced from your CRM (e.g. Salesforce) or from your Google Workspace.

From your Google Workspace, you will have calendar meetings & emails synced that are associated with the selected record.

You can sync records that are being sourced from your Google Calendar to your CRM by opening the detailed side panel and clicking the CRM icon.