A few years ago I recall using the terminal and reading a tutorial in the Linux manual (using man) on how a computer worked after it was turned on. It walked you through the whole process explaining the role of the BIOS, ROM, RAM and OS on this process.Which page was this, if any? How can I read it again?
Which man page describes the process of a computer turning on?
linux;boot;man
You're thinking of the boot(7) manual (man 7 boot) and/or the bootup(7) manual (man 7 bootup).  Those are the manuals I can think of on (Ubuntu) Linux that best fits your description.These manuals are available on the web (see links above), but the definite text is what's available on the system that you are using. If a web-based manual says one thing but the manual on your system says another thing, then the manual on your system is the more correct one for you. This goes for all manuals.See also the See also section in those manuals.This other question may also be of interest:  How does the Linux or Unix  /  get mounted during bootup?For a non-Linux take on the boot process, the OpenBSD first-stage system bootstrap (biosboot(8)) and second-stage bootstrap (boot(8)) manuals, followed by rc(8), may be interesting.