Apple has completely redesigned and reorganised startup modes in M1 Apple Silicon Macs. Instead of having to memorise all the startup key combinations used by Intel Macs, different modes and features are accessed in a structured and logical order. Some, such as Safe Mode, may not be as convenient to access, but follow a logical sequence. Here’s a quick reference.

Diagnostics Mode, to check your hardware
Press and hold the Power button until the display shows Loading Startup Options, then release it. Once the Startup Options screen appears, press and hold the Command and D keys until your Mac restarts and the Diagnostics Loader menu appears in the main menu bar.

Recovery Mode, to check startup disk, reinstall macOS, restore from a backup, change security settings
Press and hold the Power button until the display shows Loading Startup Options, then release it. This takes you to the Startup Options screen. Select the Options icon, then click Continue underneath it.
Tools provided include:

• main window – Restore from Time Machine Backup, Reinstall macOS, Safari, Disk Utility
• Apple menu – Startup Disk, Restart, Shut Down
• Utilities menu – Startup Security Utility, Terminal, Share Disk
• Online help is available when you open Safari.

Fallback Recovery Mode, identical to regular Recovery Mode except that Startup Security Utility isn’t available
Press the Power button twice in rapid succession, and on the second of those presses hold the button until the display shows Loading Startup Options, then release it. This is invaluable if regular Recovery Mode is unavailable for any reason. This only becomes available after a Mac has had a macOS update: see this article for further details.

Safe Mode, to flush user caches and disable third-party extensions
Press and hold the Power button until the display shows Loading Startup Options, then release it. This takes you to the Startup Options screen. Select the disk which you wish to boot from in Safe Mode, then press and hold the Shift key and click Continue in Safe Mode underneath it.

Startup Manager, to select which volume from which to boot
Press and hold the Power button until the display shows Loading Startup Options, then release it. This takes you to the Startup Options screen. Wait until all bootable disks have loaded into the list. Select the disk you want to boot from, then click Continue underneath it.

Set default startup volume
Press and hold the Power button until the display shows Loading Startup Options, then release it. This takes you to the Startup Options screen. Select the disk which you wish to make the default boot disk, press and hold the Option key and click Always Use underneath it.

Restore macOS from a previous backup (no longer applicable)
Press and hold the Power button until the display shows Loading Startup Options, then release it. This takes you to the Startup Options screen. Select the Options icon, then click Continue underneath it. In the macOS Recovery window select your user icon and click Next. Enter your password in the dialog, select Restore from Time Machine and click Continue. Click Continue in the next window. Time Machine System Restore will then list Restore Sources. To restore from a previous backup, select your backup disk and click Continue. Then select the backup which you wish to restore. To revert to the previous version of macOS, select the last snapshot containing that version, then click Continue. If that backup contains a snapshot of the System volume, a dialog will then invite you to install that version of macOS, which you should accept. The snapshot should then be restored, and your Mac will reboot from that version of macOS. However, as of 11.2.1 neither the macOS updater nor Time Machine keep snapshots of the System volume, so this feature no longer works, and just puts you back to installing macOS instead.

Verbose Mode, to see details of startup process
A Recovery Log is available from the Window menu in Recovery Mode. Normal Unified log records of the boot process start with the initialisation of kprintf by the kernel. Normal verbose mode has been achieved by adding a boot argument of -v in the NVRAM:
sudo nvram boot-args=”-v”
However, this no longer appears to work in macOS 11.2 and later. You should also be very careful when working with NVRAM settings on an M1 Mac: any errors or problems may require a complete system reinstall, apparently.

Reset SMC/NVRAM
Not available. NVRAM contents are listed in System Information under Software > Logs > NVRAM contents, and can still be edited using the nvram command in Terminal. M1 Macs lack a discrete SMC which could be reset in the way that they were previously, and the only solution to NVRAM problems may be a complete system reinstall.

DFU Mode, to recover from firmware or other system/update problems
Shut down, connect Macs, and follow the detailed instructions provided here and here by Mr Macintosh or here by Apple, and my own additional contribution here.

Target Disk Mode, to connect to another Mac
Connect Macs using a USB, USB-C or Thunderbolt cable. On the Target, enter Recovery Mode and use the Share Disk command in the Utilities Menu.

Single-User Mode / Terminal in Recovery Mode, to run command tools in minimal macOS
Not available (see comment below for an elaborate method for engaging a much less convenient alternative). Start up in Recovery Mode, and open Terminal from the Utilities menu.

Updated 13 December 2021. Thanks to CMMChris for the information on Verbose Mode, to NinjaTurtle45 for information on the most complex single-user mode on the planet, and to SoftRAID for the instructions for rolling back to a System snapshot.