Conservation of momentum in a collision between particles can be understood from [NCERT Exemplar]

We know that, for a system, Fext =dpdt              (i.e. from Newton's second law)If Fext = 0, dp = 0 ⇒ p = constantHence, momentum of a system will remain conserved, if external force on the system is zero.In case of collision between particles, equal and opposite forces will act on individual particles as per Newton's third law.Note We should not confuse with system and individual particles. As total force on the system of two particles is zero, but force acts on individual particles.