So I've already asked a similar question on Quora and the askphysics forum on Reddit, but haven't gotten many helpful answers. I did get one good answer that said that light SOMETIMES gains mass in mediums but not always, but didn't explain why. And I got one other good answer talking about the difference between phase velocity and group velocity, which is a distinction I've heard come up before when I've asked questions about why/how light can slow down in a medium, but I don't really understand when one is relevant vs the other or what it has to do with whether light in a medium massive or not.
Here are the details of the question as I posted it on Reddit: This PBS Space Time Video explains that mass is simply the emergent property that causes collections of individual massless particles to gain inertia and stop traveling at lightspeed. They said it's due to the confinement of energy, hence why most of an atom's mass isn't from the masses of the individual fundamental particles that make it up, but from the confined energy of the bonds.
They summed it up as saying that, by themselves, all fundamental particles would be massless and travel at $c$, but once they start interacting with each other, which results in confined energy, mass shows up. This video says that, e.g. electrons are always interacting with things because the electron field is always interacting with the Higgs field, and hence electrons always have mass. My impression is that the EM field (AKA the photon field) doesn't interact with the Higgs field and hence individual photons don't have mass.
So my question is, if a larger collection of photons are all interacting with something else, such as glass, shouldn't those interactions result in confined energy and hence mass, even if the individual photons are still massless?
