Clip 1 - 8 December 2017 - Law 17

Whose ball?

Jonny Sexton of Ireland kicks a free kick high downfield against Argentina who are playing in black for some reason. Ramiro Moyano of Argentina catches the ball and is immediately grabbed by Jacob Stockdale if Ireland who shunt him back several metres, the ball still comes back to Martín Landajo who passes to Agustín Creevy ion his left. James Ryan tackles Creevy who goes to ground. The ball comes back to Landajo who kicks a high box kick. Sexton catches it but is immediately caught by Moyano and Tomás Lezana. Sexton stays upright and other players gather around. The referee announces the maul, which it is.

The maul stops moving and the referee immediately awards a scrum to Ireland, because Sexton had caught a kick and the maul was formed on him.

Law 17.6 Unsuccessful end to a maul
(b) A maul ends unsuccessfully if the ball becomes unplayable or collapses (not as a result of foul play) and a scrum is ordered.
(h) Scrum after a maul when catcher is held. If a player catches the ball direct from an opponent’s kick, except from a kick-off or a drop-out, and the player is immediately held by an opponent, a maul may form. Then if the maul remains stationary, stops moving forward for longer than 5 seconds, or if the ball becomes unplayable, and a scrum is ordered, the team of the ball catcher throws in the ball.
‘Direct from an opponent’s kick’ means the ball did not touch another player or the ground before the player caught it.

Sexton caught the ball directly from Landajo's kick.
Sexton is immediately held by opponents.
A maul formed.
The maul remains stationary.
The referee awards a scrum.
Ireland put the ball in.

That is step by step according to (h) above. advert