How computers display raw, low-level text and graphics

Problem

My ever-growing interest in computers is making me ask deeper questions, that we don't seem to have to ask anymore. Our computers, at boot, as far as I understand it, are in text mode, in which a character can be displayed using the software interrupt when . We've all seen the famous booting font that always looks the same, regardless of what computer is booting. So, how on earth do computers output graphics at the lowest level, say, below the OS? And also, surely graphics aren't outputted a pixel at a time using software interrupts, as that sounds very slow? Is there a standard that defines basic outputting of vertices, polygons, fonts, etc. (below OpenGL for example, which OpenGL might use)? What makes me ask is why OS' can often be fine without official drivers installed; how do they do that? Apologies if my assumptions are incorrect. I would be very grateful for elaboration on these topics!