Command (⌘)-R: Start up from the built-in macOS Recovery system. Or use Option-Command-R or Shift-Option-Command-R.
Option (⌥) or Alt: Start up to Startup Manager,
Option-Command-P-R: Reset NVRAM or PRAM.
Shift (⇧): Start up in safe mode.
D: Start up to the Apple Diagnostics utility. Or use Option-D.
Command-S: Start up in single-user mode.
T: Start up in target disk mode.
Command-V: Start up in verbose mode.
Eject (⏏) or F12 or mouse button or trackpad button.