Hcfyk

The are both examples of self-organizing systems or of self-organization.

See wikpedia: self-organization

Another term used is swarm intelligence.

See wikipedia: swarm intelligence

I might call them a collective:

collective n

1:  a collective body :  group

2:  a cooperative unit or organization; specifically : collective farm

That may imply some individual agency and intention to contribute to the whole, which wouldn't be quite right for fish or neurons. A more general term would be a system:

system n

: a group of related parts that move or work together

: a body of a person or animal thought of as an entire group of parts that work together

: a group of organs that work together to perform an important function of the body

... and the behavior of the whole an emergent property:

emergent property n

a property which a collection or complex system has, but which the individual members do not have

A metaphor in living organism is siphonophores (an example of which is the Indo-Pacific Portuguese Man-of-War (Physalia utriculus), or blue bottle):

a class of marine animals belonging to the phylum
Cnidaria. Although a siphonophore appears to be a single organism,
each specimen is actually a colony composed of many individual animals
called zooids, all of which have a specific role for survival.

So each of siphonophore or zooid is a word that describes many individuals working together to form a whole (yet maybe not THE one the OP is looking for).

Or maybe simply a network (an interconnected system of things or people)?

How about macrocosm, as in:

• "The school of fish acted as a macrocosm of the individual fish."


• "The brain acts as a macrocosm for trillions of neurons working in concert." or "Trillions of neurons work in concert, serving as a microcosm of the larger brain".

From Wikipedia:

Macrocosm and microcosm refers to a vision of cosmos where the part (microcosm) reflects the whole (macrocosm) and vice versa...a macrocosm is a social body made of smaller compounds."

I like the self-organizing system answer.

As a metaphor I would also suggest a superorganism also spelt supraorganism.

The school of fish acts as a superorganism comprised of the individual
fish.

The brain is a supraorganism comprising trillions of neurons working
in concert.

From the Wikipedia article:

The term was coined in 1789 by James Hutton, the "father of geology",
to refer to Earth in the context of geophysiology.

Some scientists have suggested that individual human beings can be
thought of as "superorganisms"; as a typical human digestive system
contains 1013 to 1014 microorganisms whose collective genome, the
microbiome studied by the Human Microbiome Project, contains at least
100 times as many genes as the human genome itself.

I'd suggest, school

: a large number of fish or aquatic animals of one kind swimming together

M-W

: a large group of aquatic animals, especially fish, swimming together; a shoal.

[Middle English scole, from Middle Dutch; see skel-1 in the Appendix of Indo-European roots.]

AHDEL

The neural pool contains many schools of neurons. Cast your net on the right side of the brain.

Expand Your Mind, Improve your Brain - Chapter 24

Alternately, you might consider tank, as in fish tank and think tank

think tank

: an organization that consists of a group of people who think of new ideas on a particular subject or who give advice about what should be done

M-W

...and bank

A mound, pile, or ridge; a group or series of objects; an amount or stock of money; a batch of paper money.

Dictionary of Collective Nouns and Group Terms

A neural network is a bank of neurons operating in parallel and interacting together to learn autonomously [...]