The circuit board carries a copyright label for 'insyde H2O' which lets us know the BIOS used.
On power-up the DEL key takes you to the startup options. This brings up a screen labelled 'Front Page' and offering five choices:
Continue >Boot Manager >Device Management >Boot From File >Setup Utility
Boot Manager: brings up a display of all bootable devices that it finds and lets you select which one to boot. The boot devices have to be in UEFI format to be recognised, there is no support for legacy booting.
Device Management: does nothing for me.
Boot From File: On my system, with the eMMC flash wiped clean and booting from a SanDisk Cruzer Blade USB drive plugged into the bottom USB port on the front, it shows:
NO VOLUME LABEL, [PciRoot(0x0)/Pci(0x8,0x1)/Pci(0x0,0x3)/USB(0x1,0x0)]
Setup Utility: takes you into the InsydeH20 BIOS. In the Linux dmesg output I find:
DMI: Igel Technology GmbH M350C/M350C, BIOS M350C V:3.3.13A-10312019 10/31/2019
As is often the case with thin clients there are only a limited number of settings to play with in the BIOS - enable/disable Secure Boot, various 'wake up' triggers, boot options such as USB & PXE enable/disable...
My unit was wiped clean by the seller but I believe would have been running Igel's OS11.