According to our current understanding in particle physics, quarks and leptons are considered elementary particles, which means they are not believed to be composed of smaller constituents. They are considered fundamental building blocks of matter.
Quarks are the constituents of hadrons, which are composite particles that include protons and neutrons. Quarks come in six different flavors: up, down, charm, strange, top, and bottom. Protons and neutrons, for example, are composed of three quarks each. For instance, a proton consists of two up quarks and one down quark, while a neutron consists of one up quark and two down quarks.
Leptons, on the other hand, do not participate in the strong nuclear force and do not have substructure. There are six known leptons: the electron, muon, tau, and their associated neutrinos. The electron, for instance, is a fundamental particle and is not composed of any smaller particles.
It's important to note that quarks and leptons are considered point-like particles in the Standard Model of particle physics, meaning they are described as having no size or internal structure. They are treated as elementary particles in mathematical models and are not believed to be made up of anything smaller.