In a closed system with no external forces, the total momentum remains absolutely constant, no matter how the internal parts interact or collide.
Momentum ($\vec{p}$) is the product of an object's mass and velocity. In a perfectly elastic collision, objects bounce off each other without losing any kinetic energy. Both total momentum and total kinetic energy are conserved.
A classic desk toy where lifting and releasing one ball causes exactly one ball on the opposite end to swing outward. The momentum transfers instantly through the rigid steel spheres, perfectly preserving both $m$ and $v$.
Momentum is always conserved in a closed system, even if the kinetic energy is not. In an explosion (or objects pushing apart), a single object at rest ($p=0$) splits into pieces whose combined momentums must rigidly cancel each other out to remain zero.
When a cannon fires a cannonball forward, the heavy cannon physically recoils backward. Because the initial momentum was zero, the forward momentum of the light but incredibly fast cannonball is exactly equal and opposite to the backward momentum of the massive, slower cannon.
Angular momentum ($L$) is the rotational equivalent of linear momentum. It depends on an object's moment of inertia ($I$) and its angular velocity ($\omega$). Just like linear momentum, in the absence of external twisting forces (torques), it remains constant.
When a figure skater spins with their arms fully extended, they have a large moment of inertia ($I$). When they suddenly pull their arms and legs in tightly, their radius decreases dramatically. Because $L$ must stay constant, their angular velocity ($\omega$) spikes, making them blur into a super-fast spin.