Hangarback Walker enters and dies as a state-based action due to having 0 toughness, triggering Mortuary and High-Society Hunter.
Resolve the Mortuary trigger, putting Hangarback Walker from your graveyard onto the top of your library.
Resolve the High-Society Hunter trigger, causing you to draw a card, thus drawing Hangarback Walker from your library.
Repeat.
This clever loop puts a zero-mana Hangarback Walker right back into your hand by using its state-based demise as the engine for the rest of the board. As the automaton hits the graveyard, Mortuary instantly redirects it to the top of your library, perfectly setting up High-Society Hunter's triggered ability to draw it the moment it dies. Because the Hunter sees each death trigger as a nontoken creature perishing, you get to continuously cycle the Walker to draw your entire deck while racking up infinite creature leaves-the-battlefield, enters-the-battlefield, and storm counts along the way.