Question: What are quarks and how do they work? Are they the smallest "thing" to be discovered?

James Smallcombe answered on 26 Jun 2020:

Quarks are indeed one of the smallest particles to be discovered. As far as we can tell there is nothing inside a quark, they are point-like (have no volume, no inside) but they have mass, charge and other physical properties that control how they interact with the world and stick together.
Quarks always stick together in groups. A group can stick together so strong it looks and behaves like a single particle, the most common being groups of 3 making protons and neutrons.

We call the smallest things “fundamental particles”, as well as quarks there are also electrons, neutrinos and “force carrying” particles.
As none of them have volume I suppose the electron-type-neutrino is the “smallest” as it has the smallest mass.