You're equivocating environments and hosts.

Hosts do the replication for the viruses.

Environments do NOT do the replication for the fish.

Single-cell organisms replicate.

Single cells of multi-cellular organisms replicate.

Shoot me in the head and all my cells will eventually stop replicating.

To a cell in my body, is the body a host or an environment? Is the distinction useful?

I would say neither. Your body is a colony, like an ant colony or bee colony. In all instances, most individuals can't start a colony of their own but it's limited to a set of queens/gametic cells.

As for shooting into the head, some multicellular life forms can actually heal from that. From planaria you can remove any part of their bodies, it will grow back. Shooting into an ant colony won't do much harm to them either, nor is removing the branch of a tree. It's only a property of animals I'd say who have given up this property (for the most part) in exchange for more complex metabolic systems. (heart, nervous system, etc)

Just because it happens outside the ‘parents’ body doesn’t mean the environment does it anymore than a mother being only a filter around the womb and the environment.

An egg is a complex life that can reproduce the whole organism (and actually with energy stored INSIDE, that’s why they are great nutritios value for others)

The distinction between environment and organism here seems a little arbitrary. The organism’s cells have membranes, but they’re permeable and the organism can’t do its job without an interaction between itself and the environment. Even with the energy stored inside, it requires warmth and atmospheric pressure. I suspect there are any number of counter examples of life forms that challenge the simplicity of this formulation. You could just as easily argue that each cell is a life form unto itself, right?

Isn’t each cell a life form unto itself? Like there are cell lines from cancer cells that has been living outside their ‘host’ long after the human they were aquired from died (HeLa) They just specialize in an specific environment created and maintained by cells with the same genetic material

We are all just environments for ribosomes.

You've not understood the depth of the original comment's thinking.

this is certainly not the mainstream way of thinking; it's certainly not specific enough.