In the Adobe Creative Cloud App, "Apps" tab at the very end of the "Lightroom Classic" line, click the three dots and then choose tutorials. You will be looking for Using Masking.
Ken Seals - Nikon Z 9, Z 8, 14mm-800mm. Computer Win 11 Pro, I7-14700K, 64GB, RTX3070TI. Travel machine: 2021 MacBook Pro M1 MAX 64GB. All Adobe apps.