After completing a couple of Dynamax Adventures, you can then start to participate in Endless Dynamax Adventures by talking with the doctor character within the Max Liar. These types of Dynamax Adventures work just like regular Dynamax Adventures, except that the adventure continues endlessly until your team has suffered enough faints to be knocked out of the Pokemon Den. You still choose a Rental Pokemon to begin with, get to pick what path of Dynamax Pokemon to battle, and you still face off against a Legendary Pokemon in every fourth battle.
Outside of the main difference of these Dynamax Adventures continuing endlessly until you lose enough knockout lives, there are a few smaller differences as well. For one, you will not be able to capture any of the Legendary Boss Pokemon. You will still be able to capture regular Dynamax Pokemon to aid in replacing your weakened Rental Pokemon for longer survival, but you will never have the option of doing so with the Legendary Pokemon. Furthermore, you will gain loads more of Dynite Ore in Endless Dynamax Adventures, especially the further and further you can go in a streak.