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