While your second sentence is definitely true, I disagree with your first. Developing muscle strength is partially caused by training your nerves to better engage your muscle fibers; however, simply adding fibers aka raw muscle mass is the other main component of gaining strength. I would argue it's the predominant factor.

You're both right. Improved muscle fiber recruitment is the primary way beginners gain strength. This is a fairly fast process, but plateaus off quickly. After the so-called "noob gains" are through, growing more muscle is the primary way to gain strength.

My knowledge on the subject is a bit out of date as I'm not in a serious training for more than a decade, but if I remember correctly explanation from my coach there're 3 main aspects of strength: nerve paths to activate the muscle fibers, the volume of muscle cells (when you get "big muscles" you actually don't increase significantly the number of cells, but their volume), and the capability of muscle cells to use and store glucose from the bloodstream (cellular uptake, which is also increased by regular training).

This is largely true. Though, as with anything in the body, the entire system reacts. Bones will change density, the circulatory system also changes in relation to the increased mass, the cerebellum will re-wire due to the increased muscle mass and usage cases, the reproductive systems may change depending on many factors, the pancreas will change it's output of glycogen, etc.

When it comes down to building muscle mass and 'dexterity', the physiology is fairly well understood. For most athletes today the major issues are in repair/strain and consistency. AKA: Sleep well, eat well, train well, in that order. It is theorized that a lot of the recent gains in MLB pitching are due to improved tendon repair chemical therapies and traditional steroids that focus on bulk (no data here, I think I read it on 538 a few years back)

Adding fibres. i.e. adding cells is hyperplasia and it's currently unknown if this occurs naturally in adult humans. Most visible growth can be attributed to hypertrophy, where the cells grow bigger.

My apologies. I was referring to "myofibrils" which are the strands within a muscle cell. So I meant to say that hypertrophy is the mechanism of growth. But you are correct, these are simply parts of a single cell and I mistakenly used hyperplasia.

If I'm not mistaken, muscle fibers can only fire maximally--a muscle fiber either fires (maximally) or it doesn't fire. Isn't muscle fiber recruitment what differentiates delicate, precise motions from gross, explosive motions?

Yes and no. Individual fibers are either On or Off, but individual fibers are grouped into bundles, and any one muscle is composed of many bundles. When you flex a muscle to move a thing, your brain only recruits some of the bundles. The exact amount is determined by some combination of your willpower, focus, training level, and adrenaline.

As you train (that is, try really hard to lift something heavy), you teach your brain that you're going to be lifting heavy things a lot going forward, which makes your brain recruit more muscle fibers when you actually go ahead and lift the thing.

So you're right in one sense, and incomplete in another.

That can't be everything though because otherwise your body wouldn't physically change when you train and they clearly do.

Also why wouldn't evolution just make us all strong all the time if it only requires neurological changes?

Strength is generally an evolutionary drawback, in most cases the benefits of extra strength doesn't outweigh the calorie cost of building maintaining it.

For very costly things like strength and brainpower, evolution is ensuring that we have as much as we require, and not a single bit more; and has all kinds of processes for actively reducing strength that you had unless you have both extra calories and the demonstrated need to use that strength; because it saves calories.

It’s not everything. See muscle hypertrophy: https://en.m.wikipedia.org/wiki/Muscle_hypertrophy

Probably a combination of not needing to be very strong (we don't use raw strength to hunt very often) and a metabolic cost.

Metabolic cost is a big factor.

It's one of the theories of why homo sapiens out competed neanderthal basically everywhere, we can survive the lean times better.

Iirc they required about twice the average intake of an anatomically modern homo sapiens.

because the more fibers you use for small movement the more energy you consume and higher the risk of actual damage

Source? This is very interesting

And partially incorrect. This is indeed the case for the first few weeks of strength training. In the beginning your body learns to recruit and better utilize the existing muscles you have. But after that the long and slow process of building new muscle mass starts. The CNS adaptation( i.e. the first part) never goes away. That's the reason why if you have trained in the past for at least a couple months and now you restart your training your new starting weights will be higher than your old ones.