> what is the grammar/rule around what gets pluralized and what doesn't? How does one know that "code" can refer to a single line of code, a whole file of code, a project, or even the entirety of all code your eyes have ever seen without having to have an s tacked on to the end of it?

Well, the grammar is that English has two different classes of noun, and any given noun belongs to one class or the other. Standard terminology calls them "mass nouns" and "count nouns".

The distinction is so deeply embedded in the language that it requires agreement from surrounding words; you might compare many [which can only apply to count nouns] vs much [only to mass nouns], or observe that there are separate generic nouns for each class [thing is the generic count noun; stuff is the generic mass noun].

For "how does one know", the general concept is that count nouns refer to things that occur discretely, and mass nouns refer to things that are indivisible or continuous, most prototypically materials like water, mud, paper, or steel.

Where the class of a noun is not fixed by common use (for example, if you're making it up, or if it's very rare), a speaker will assign it to one class or the other based on how they internally conceive of whatever they're referring to.