Mac startup key combinations.

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 while starting up. If your Mac is using a firmware password, you're asked to enter the password.

Option (⌥): Start up to Startup Manager, which allows you to choose other startup disks or volumes, if available. If your Mac is using a firmware password, you're asked to enter the password.

Option-Command-P-R: Reset NVRAM or PRAM. If your Mac is using a firmware password, it ignores this key combination or starts up from macOS Recovery. 

Shift (⇧):  Start up in safe mode. 

D: Start up from the built-in Apple Hardware Test or Apple Diagnostics utility, depending on your Mac model. Or use Option-D to start up from this utility over the Internet.

N: Start up from a compatible NetBoot server, if available. To use the default boot image on the NetBoot server, hold down Option-N instead. Computers that have the Apple T2 Security Chip don't support this startup key. 

Command-S: Start up in single-user mode. This key combination requires macOS High Sierra or earlier. 

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.

Cr. https://support.apple.com/ro-ro/HT201255