The mask is automatically applied. You don't have to do anything to apply it. To return from masking to the non-masking part of LrC, you can click on the Edit icon or click on the Masking icon.
When you see changes in the user interface, you should always read the "What's New", which will appear automatically the first time you run the new version, and is always available vvia the menu command Help->What's New.
Full coverage of the new masking at https://helpx.adobe.com/lightroom-classic/help/masking.html