To enter BIOS for most System76 laptops, hold the F2 key immediately after turning your computer on. For the boot menu, hold down the F7 key. For older laptops, hold the F1 key.
To enter BIOS for desktops (besides the Meerkat), hold down the DEL key. To show the boot menu, hold the F8 or F12 key.
For the Meerkat, hold the F2 key for the BIOS, or the F10 key for the boot menu.
The newest Darter Pro (darp6) and Galago Pro (galp4) contain System76 Open Firmware with Coreboot. The BIOS menu can be accessed by tapping the ESC key immediately after powering the system on. The following screenshots show what the menus look like: