This Sharepoint tutorial will demonstrate how you can add a calendar to a SharePoint Online site. The approach used in this tutorial creates a Sharepoint List and applies a calendar view to it. The list can then be added to pages on the Sharepoint Online site and can also be added to channels in Microsoft Teams. One major limitation with this approach is that the calendar cannot be synchronized with Microsoft Outlook as with the legacy calendar webpart.
How To Add A Calendar To A SharePoint Online Site