Can't create OS X El Capitan bootable USB on Mac M3

2012 MacBook Pro should have Internet recovery in its ROM.

In that case, you should be able to invoke recovery on the 2012 MacBook Pro with:

 Shift-Option-Command-R

You are requesting the "shipped-in-the-box" version, which is tagged ONLY with the serial number of your Mac, and does not require any Apple-ID to be already working on your Mac.

once you have ANY working MacOS, things get a little easier.

If you're having trouble creating a bootable USB for OS X El Capitan on a Mac M3, one straightforward solution is to borrow an old Intel Mac. Older Intel-based Macs natively support creating bootable USB drives for earlier versions of macOS, including OS X El Capitan, using tools like Disk Utility or Terminal.

By using an Intel Mac, you can easily create OS X bootable USB without compatibility issues that may arise with Apple Silicon Macs, and then use it to install El Capitan on the desired machine. This method bypasses the limitations of the newer architecture and simplifies the process.

How about booting into Internet Recovery Mode using Command + Option + R to attempt to boot into the online macOS 10.15 Catalina installer?

Do you have access to another working Mac from 2007 to mid-2020? If so, then you can use that other Mac to create a bootable macOS USB installer. The exact model of the other Mac will determine which macOS USB installer you can create.

If you can install any version of the OS on the old Mac, just copy the .DMG over to it (with the aid of a USB flash drive)and open the image and run the installer application.

No need for a bootable installer then.

the MacOS 10.11 El Capitan .dmg is a Disk Image file. it only contains a .pkg file.

when opened, the .pkg file PLACES the "Install MacOS ... El Capitan ...app" in the /Applications folder

YOU Can NOT proceed to test-Install El Capitan on an M3 Mac.

None of this should invoke an Installer at all.

Once the 'Install MacOS El Capitan...' app is in the /Applications folder, you can COPY the Terminal command from the article on making a bootable Installer and PASTE it Directly to the Terminal command line, provided you named your USB-Stick drive MyVolume.

I agree with Grant Bennet-Alder. So even though you can't create OS X El Capitan bootable USB on Mac, you're on the right track by preparing the installer app, because that's really the key step. After that, pasting the Terminal command is straightforward and only requires following the script exactly, which you're set to do once you have the installer in the right place. Just remember that you still need to borrow that old Intel Mac or use another method for actually creating the bootable USB since the M3 won't support that directly.

Before creating OS X El Capitan bootable USB on Mac M3. You have to make sure you download the correct version of El Capitan from the App Store. I ended up with a weird version once and thought I was good to go. Double-check that your download is legit and matches what your Mac requires, or else the USB might not be recognized at all.

If you're stuck trying to create a bootable USB for OS X El Capitan on an M3 Mac and you have to go the route of borrowing an old Intel Mac, but if you're not super familiar with older macOS versions, navigating through the Disk Utility or Terminal on that Intel Mac can be a bit of a learning curve. You might end up spending more time just trying to figure it out than actually creating the bootable USB.