What is/might be the basic component that makes up elementary particles (like electrons and quarks)? Is this the stuff concerned with “strings” (string theory)? Can anyone explain it in layman's terms?
This is the table of elementary particles
The very successful theory , the standard model of particle physics, uses this table axiomatically, i.e. as a foundation stone of the theory. These are a distillation of measurements, and the answer is: they are point particles, i.e. not composite, and there is no further complexity.
String theory embeds the standard model in its mathematics. It changes the dimension from a point to a string, but still there is no complexity to the string. Each particle is assigned a vibrational niche on the equations of the vibrating string, and no extra compositional complexity to individual particles exists, even in string theories.
There have been proposals for complexity in the particles in the table, look at preons, they are off main stream research because of the success of the standard model approach. The particle data group lists the searches for limits of compositeness.