Yeah, I had the same assumption when I got my R6C—thought that plastic window meant an IR receiver was built-in. Unfortunately, it's just for aesthetics. I did try the Matrix IR Receiver you linked, and while it can technically be wired to the board, it doesn’t fit cleanly in the case without some D...

You can't find where BIOS/Boot settings are stored but need to access them to boot from a USB drive containing a backed-up Ubuntu OS. You're wondering where the settings are physically stored and how to access the Boot Menu during startup to select the USB drive as the boot device.