From my experience and reading Piter’s explanation, the SOC looks for the bootloader (u-boot) in eMMC followed by SD card.
What happened was that I downloaded and flashed an Armbian build to the eMMC card, but it failed to boot. Was getting blinking red led and no output via HDMI. Hoping to correct this, I inserted my working Radxa SD card, but the Rock Pi still failed to boot.
Which means that if eMMC card contains an incompatible/incorrect/corrupted bootloader, the system will fail to boot. After removing the eMMC, was able to boot from SD card. Then I flashed the current version of Armbian to eMMC which worked.
Some boards like the RockPro64 offer an option to disable eMMC while booting by shorting pins via jumper. Will try Piter’s suggestion to pause bootloader when I have a functional USB TTL debug cable.
Forgive me if I use some incorrect terms, am new to this.
I bought v1.3 of Rock Pi 4 rather than v1.4 since it was cheaper and I read about some problems with booting after flashing incorrect SPI image. I realize now that it was a mistake. A correctly flashed SPI can correct this problem and allow flashing all kinds of images to the eMMC without worrying if it will boot.