Processes in Microsoft 365 for setting up Office apps, redeeming product keys, and activating licenses.
If you have the correct role (SharePoint admin or GA), you can go to the SharePoint admin center, click on 'Active Sites,' then open the site. From there, hit the 'edit' button on the right side and you can control the navigation and the look/feel of the site. There are also several templates you can choose from to help things along.
For more advanced design, you can use SharePoint designer.
Here's a quick and dirty walkthrough: