What commands do I enter for command s keys in black screen?

All keyboard shortcuts presume existing keyboard of the notebook computer, or an external USB keyboard is attached. Bluetooth keyboards may not support boot key sequences. Command key is the same as the Windows logo key on keyboards not made by Apple. Alt key should be used in place of Option key for non-Apple keyboards.

command-S at startup brings up single user mode (a black screen with text and a prompt) if you hold those two keys down when you power up the computer, and let go of the power button. If nothing comes on the screen, and it is the only that is part of the laptop, or iMac, or external display for Mac Mini, Mac Studio, or Mac Pro, then there is something wrong with the video signal of the computer or the storage of the computer. Single user mode depends on a functional installation of the Mac OS on the computer.

command-R offers recovery, to bring up the existing OS recovery on Intel Macs from July 2011 to November 2020. Only works if an existing OS is properly installed.

command-option-shift-R at startup brings up internet recovery as long as the Mac is hooked up via ethernet, or thunderbolt-2 or 3 adapted ethernet on Intel Macs. Available to all July 2011 and later Intel Macs.

Use macOS Recovery on a Mac with Apple silicon - Apple Support is for Apple Silicon.

In recovery the Terminal offers the same functionality as single user mode but so does DIsk Utility's repair disk function, which most people try to use in single user mode by typing

/sbin/fsck -fy

/sbin/mount -uw

and exit.

Recovery though offers additional options to erase the drive or reinstall the existing or previous OS depending on what's available on the computer's drive. If the drive can't be properly erased, you can connect an external hard drive and format it APFS or HFS Extended GUID using Disk Utility.