https://support.apple.com/en-us/HT201255
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 prompted to enter the password.
Option (⌥) or Alt: Start up to Startup Manager, which allows you to choose other available startup disks or volumes. If your Mac is using a firmware password, you're prompted 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. Disabled when using a firmware password.
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. Disabled when using a firmware password.
N: Start up from a NetBoot server, if your Mac supports network startup volumes. To use the default boot image on the server, hold down Option-N instead. Disabled when using a firmware password.
Command-S: Start up in single-user mode. Disabled in macOS Mojave or later, or when using a firmware password.
T: Start up in target disk mode. Disabled when using a firmware password.
Command-V: Start up in verbose mode. Disabled when using a firmware password.
Eject (⏏) or F12 or mouse button or trackpad button: Eject removable media, such as an optical disc. Disabled when using a firmware password.