It's best practice to only sync folders you use regularly so your machine is not consuming resources syncing files you may not need regular access to.
1
Navigate to the folder in SharePoint that you wish to sync.
2
Click "Sync"
3
A OneDrive window will popup momentarily and disappear
4
Click "Close"
5
Open Open File Explorer
6
You will see your company name with a building icon next to it. Click the ">" to expand it.
7
You will now see the folder listed that you choose to sync.
The folder maybe empty when you initiate the sync. depending on the size of the folder and quality of your internet connection it could take a few minutes for subfolders and files to appear.