After Raspberry PI 4 firmware upgrade 15 June, I can boot PI OS on USB and yesterday night after a few attempts and some kernel panics (caused by enable_uart=1 inside the config.txt file in the boot folder) I managed to get a good working version of Manjaro XFCE 20.06. One of the issues was that some of the files in the boot folder were no longer in sync with the 15 June beta firmware update so I had to retrieve old .elf and .dat files, replace them in the boot folder and eventually I got it all working.
This isn't the case with Ubuntu Server (Arm) 20.04. The Downloaded image fails to boot from USB and the files trick didn't work. It doesn't detect USB, doesn't detect network either and then it just gets stuck at a rudimentary prompt in which I can't type because apparently USB doesn't work.