How do I access BIOS on my MacBook Pro (13 Inch - middle 2012)?
The closest equivalent is holding option at start. That runs the firmware that allows you to select network boot or external or internal bootable OS. 

There is no open firmware anymore where you can enter commands, so you can just reset NVRAM or let the option boot read the current NVRAM settings. Anything you would set in bios, you can write to the non volatile ram from any OS X booted OS and then those variables and settings are applied next boot. 

There is no bios and no concept of a user navigable hierarchy of settings to adjust before any OS loads to Apple hardware. Most of what you might do “in BIOS” isn't possible.