iMac Retina 5K 27" 2017 model with Big Sur OS and external drives available.
I have downloaded/ verified the ISO I have used Etcher to create an installer USB and a CD.
Both boot up using the option boot to select process NIETHER gets me the Try-or-Install splash screen --- just a fleeting blue icon followed by scrolling script.
(side note, same behavior with Ubuntu ISO)
I'd really like to try out a Linux OS!