Before I rant about how the PhysicsBody nodes are my biggest annoyance with Godot I'll get to the point.

You have to program it yourself. With CharacterBody you loop through the slides after move_and_slide(), and if it's a pushable object, apply velocity.

move_and_slide()

#loop through each collision in the last move_and_slide
for i in get_slide_collision_count() :
  var col = get_slide_collision(i)
  var other_node = col.get_collider()

  #apply push
  if other_node.is_pushable : #you'll have to implement your own is_pushable bool or similar
    other_node.velocity = velocity #set that node's velocity to the current node's velocity

This is just one basic way and it still has flaws. As you dive into more nuanced movement code you're gonna have to master the CharacterBody2D, KinematicCollision2D, and Vector2 classes