Key combinations for an Intel-based Mac
Command (⌘)-R: Start up from the built-in macOS Recovery system. Or use Option-Command-R or Shift-Option-Command-R to start up from macOS Recovery over the internet. macOS Recovery installs different versions of macOS, depending on the key combination you use.
Option (⌥) or Alt: Start up to Startup Manager, which allows you to choose other available startup disks or volumes.
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 to start up to this utility over the internet.
N: Start up from a NetBoot server, if your Mac supports network startup volumes. To use the default boot image on the server, press and hold Option-N instead.
Command-S: Start up in single-user mode. Disabled in macOS Mojave or later.
T: Start up in target disk mode.
Command-V: Start up in verbose mode.
Eject (⏏) or F12 or mouse button or trackpad button: Eject removable media, such as an optical disc.
https://support.apple.com/en-us/HT201255