This is a variant of Dynamax Adventures, which–as its name implies–allows you to keep playing forever, until you lose all your lives. It’s unlocked by completing the Crown Tundra story and finishing at least 3 regular Dynamax Adventures. Once you’ve met the criteria, speak to the doctor near the cave entrance.
Unlike regular adventures, you cannot keep any Pokémon you catch. However, to balance this, you can earn more Dynite Ore than usual. An Endless Dynamax Adventure functions exactly the same as a normal one, until you defeat the Legendary Pokémon at the end of the map.
Once you’ve defeated the Legendary, the Legendary will automatically run away, to prevent you from using it as a rental Pokémon. After that, your team will regain 1 life (but you can’t exceed 4 lives). Then you’ll begin at the bottom of a new map, with the same Pokémon you had, carrying over your health from the previous fight.
When you complete an Endless Dynamax Adventure, you’ll receive 1 Dynite Ore for each Pokémon you defeated, an extra 2 Ore for connecting with other players and a massive bonus if you manage to progress beyond the first map, equivalent to 200% of the ore for defeating each Pokémon. Eg. if you defeated 11 Pokémon solo, you’ll earn 33 Dynite Ore.