eli5 are neutron stars composed of neutrons, or is it quark-gluon plasma?
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It’s messier and more complicated than that.
We don’t know for sure, because of course we haven’t been able to create the conditions in a lab, but we have pretty good models of the way quarks, mesons (pairs of quarks) and nucleons (triplets of quarks) interact.
At the surface of a neutron star, there’s likely a crust of “normal” matter. It’s not necessarily the types of atoms common on earth – there will be a lot of radioactive atoms with more neutrons per atom than were used to. And of course a lot of these atoms are actually ionized (there aren’t exactly the same number of electrons attached to a given atom as protons in the atom.)
Now let’s look down at the center. This is probably neutronium. That’s basically a fluid of tightly packed neutrons. It’s probably not a quarks gluon plasma because the temperature is probably low enough that groups of 3 individual valence quarks stay close together (these 3 stick together, and those 3 stick together).
Of course it’s messy because the combination of quarks in a neutron can change into the combination of quarks in a proton by releasing some energy plus an electron and a neutrino. But theres an effect called degeneracy pressure that makes that really unlikely in a neutron star compared to a single neutron.
In between the core and crust, we think there are intermediate phases called “nuclear pasta.” because the phases look like different types of pasta.
Iirc, just below the surface you find the atoms go from “close of normal” types towards “way too many neutrons per atom”.
A little deeper and you start to see long strings of neutrons (spaghetti) mixed in. Even deeper, the strings of neutrons start forming into sheets (lasagna). There’s some other versions of nuclear pasta also. What you have at a given depth depends on the pressure. The higher the pressure, the more neutrons (compared to atoms) and also more neutrons connected together
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