A circle with a line or slash through it means that the selected startup disk contains a Mac operating system, but it's not a macOS that your Mac can use. You should reinstall macOS on that disk.
A folder with a question mark means that the selected startup disk isn't available or doesn't contain a Mac operating system.
Blank (empty) screen
It's normal for a blank screen to appear once or more during startup. Its background can be black, gray, blue, or a desktop picture. If an image doesn't appear after a few moments, make sure that your display is turned on, connected, and has its brightness turned up.
Apple logo or spinning globe
Your Mac displays an Apple logo when it finds a local startup disk, which is a startup disk built into or connected to your Mac. If it can't find one, it may look for one on your network. When using a network startup disk, your Mac displays a spinning globe instead of an Apple logo. A spinning globe also appears when starting up from macOS Recovery over the Internet. As startup continues, you should see a progress bar or , which might alternate with a blank screen several times.
If your Mac is using a firmware password, the lock icon appears when you try to start up from another disk or volume, such as an external drive or macOS Recovery. Enter the firmware password to continue.
System lock PIN code
Your Mac asks for a PIN code when it has been remotely locked using the Lost Mode feature of Find My Mac. Enter the four-digit or six-digit passcode to continue.
At the login window, enter your user account password to log in to your Mac. If FileVault is turned on, this also unlocks your disk. You might see a default desktop picture in the background, which might change to your chosen desktop picture when you select your account.