With the bootable USB plugged in, press the while holding down the Option (Alt) key.
Double-click.
shasum -a 256 /path/to/macOS\ Catalina\ 10.15.7\ 19H2.dmg download macos catalina 10157 dmg file install
or a full installer can be tricky. This guide covers the safest ways to download and install it. 1. System Requirements With the bootable USB plugged in, press the