question mark on startup screen

There are some clear ideas here:

• If your Mac starts up to a Question mark - Apple Support

https://support.apple.com/en-us/HT204323

"A folder with a 'flashing question mark' means that your startup disk is

no longer available, or doesn't contain a working Mac operating system."

The answer has two-parts, first is easy; second half below, is more involved:

"IF the question mark persists, follow these steps:

1. Press and hold the power button on your Mac for up to 10 seconds, until your Mac turns off.
2. Turn on your Mac and immediately press and hold Command (⌘) and R to start up from macOS Recovery.
3. While in macOS Recovery, use Disk Utility to repair your startup disk.
4. If Disk Utility found no errors or repaired all errors, reinstall macOS.
5. If you still need help, please contact Apple Support."

[Published Date: May 03, 2021]