Fear The Walking Dead's official explanation is that Madison's oxygen tank - a consequence of her first miraculous escape from death - spared her life by preventing smoke inhalation and suffocation.

Source: https://screenrant.com/fear-walking-dead-ending-explained-season-8-series-finale/

She's also carrying a hammer, notably smaller than Madison's but perfect for her size.

Source: https://www.cbr.com/fear-the-walking-dead-season8-episode3-recap-spoilers/

With his gun to Madison's head, Morgan can't find it in himself to pull the trigger.

Source: https://www.cbr.com/fear-the-walking-dead-season8-episode1-recap-spoilers/