To follow a SharePoint site, first launch the Teams application. For a review on how to access Teams, see the Getting Started with Teams article.
Select the Team's General Channel as shown below.
Next, select the Files tab.
Then, select Open in SharePoint.
Finally, click on the star next to the words "Not Following" to start following this Site.
You will now be following this SharePoint Site.
If you would like to sync this site with File Explorer, click on the Sync button while on the Documents tab.