FILTERING PHOTO USING STARS

Which Lightroom are you using? The cloud-centric Lightroom, or the desktop-centric Lightroom Classic?

Using Lightroom, there is no shortcut key, you need to click on the Filter icon to the right of the top-centre search bar. That will bring up the filter options, which include star ratings.

Using Lightroom Classic, the backslash key in the Library module should toggle between the filter bar showing or not showing. Once the filter bar is showing, select the Attribute tab to find the star ratings filter (or use the filter options on the right-hand side of the "breadcrumbs" bar).