2015 iMac stuck with blinking folder after factory reset attempt

Have a look here: How to reinstall macOS - Apple Support

If you erased the drive, then yes, you must reinstall the OS. You can do that by restarting the Mac in recovery mode.

Command R when powering up doesn't interrupt the blinking icon, ...

To be clear, press and hold both the ⌘ and R keys with two fingers while starting the Mac with a third finger. Keep them depressed until you see the Apple logo or "spinning globe" picture.

Once it begins to start up normally (upon hearing the startup chime) a normal startup continues and cannot be interrupted.

Release those keys only after you see the  logo or "spinning globe" picture.

Refer to How to start up from macOS Recovery - Apple Support.

Crazy Cat Guy wrote:

D.I. Johnson, thanks so much for your input. Command R when powering up doesn't interrupt the blinking icon, can't get into recovery mode, I wish, is really stuck. I'll bring it in to an Apple store.

You can also hold down the Option key to access Apple boot picker menu. From there you can press the special startup keys for Recovery Mode (Command + R) or Internet Recovery Mode (Command + Option + R).

You may need to use a wired USB keyboard to access the special startup options since a wireless keyboard may not always go ready soon enough to be recognized.

Welcome to the Apple Community.

Yes, there are solutions to this.

1. Shut down your Mac completely by pressing the Power Button for at least 10 seconds.
2. Prepare another Mac or a Thunderbolt cable.
3. Plug two Macs together. Open the Finder first. Leave it like that.
4. On Intel Macs (which is your Mac), Press and release the power button, then immediately press and hold all four of these together on the built-in keyboard: Control ⌃ on the left side of the keyboard Option ⌥ on the left side of the keyboard Shift ⇧ on the right side of the keyboard Power button.
5. Now, see if the other Mac's Finder screen shows the device in "DFU mode". If no, try again.
6. On the Finder screen, Press Revive to try to revive your Mac before the doomed state.
7. If "Revive" failed, press on the "Restore" Button. Make sure you have internet connection on the Mac.

Reference Link (Detailed Insturctions)

How to revive or restore Mac firmware - Apple Support

Good luck, and

Regards,

P.s, We don't give links or unrelated solutions!

[Edited by Moderator]