What is the "generic rule" for determining how many words to "justify" on a line in a book?

Let’s say that we have a raw book script such as:

Alice looked on the hobbit, shocked, and said: "You have eaten all our food?! Then what will we eat?" Bilbo replied: "Sorry about that. It was just so good!" Blablabla…

So, we know the font size, the font, the book page dimensions, and the margins. We have all those facts. Now, how does the person (or machine) which creates the final book page know where to "cut off" a line and start placing the words with even spaces between them?

When I read a book, all the rows of text are perfectly lined up both to the left and to the right. In the right end, there’s often an ending "-" (I forget what that is called), and most of the time, I cannot at all notice that there is more than one standard space between each word. However, there must be, or an extra fraction of a space, because they line up perfectly.

In magazines and "lesser" printed media, I can often spot (at least in the old ones from the 1990s, when I stopped reading magazines) large "gaps" which doesn’t "quite look right". This is also noticeable in web browsers where the CSS property text-align: justify (or something) has been used on a paragraph.

I’ve always wondered and tried to figure out how this is actually calculated or determined. I guess a human doing this manually, which I assume is no longer the case since several decades, could use their brain to make sensible "cut-off points" and just experiment, line by line, but if a computer has to do this automatically, it must base this on some kind of fixed logic.

What is that logic?

Does it measure in millimeters how long the current row is and keep adding more until it just about "overflows", then removes enough to account for the "dash rules" and then add equal space between each word? Is it really that "simple"? (I’ve never been able to figure out how to code this myself. I think it’s probably way beyond my abilities and likely highly complicated.)

And if this is now fully automated, how long has it been like this? Are we talking 1970s? 1960s? Even earlier? Or maybe even as late as 1990s? Some professions have been manual for a surprising amount of time, and it almost seems like a computer couldn’t produce such "perfect" book pages, but it could also be very simple and I’m just not thinking about it the right way.