In Newtonian mechanics, a particle (in my knowledge) is a point-like mass with no shape and size, deformation, rotation and internal movements, which is an idealized model of an object which does have shape and size and can have deformation, rotation and internal movements.
Newton's Laws of Motion (in my knowledge) are originally given for particles but can be extended for extended bodies, like for a rigid body.
To derive Newton's Second Law for a rigid body we can consider (as my textbook says) it to be a collection of particles.
I do not understand that when we say a rigid body is a system of particles, what are we referring to 'particles' here? Atoms? a differential element of the material? or something else?
As an extension of this question, can we consider a deformable body as a collection of particles and develop the Newton's Second Law for it as well? If yes then what will be the 'particle' in the case of deformable body?