First, you have to finish the game in the mainland, Galar, not the Isle of Armor. After you beat the main-game, Go back to to the Isle of Armor inside the Master Dojo. You will find Hop there. Walk up to him and get into a cutscene. When it is done, you go on a quest with Hop to find honey to help Urshifu G-max. You have to go in the Forest of Focus looking for Pokemon with Hop. Eventually, you realize you need to find a Vespiquen. You ride your water bike northwest of the Tower of Waters, find Hop there on a Honeycomb shaped island, do a accidental Max Raid, and get the Max Honey after you defeat the Raid. After that, collect at least 3 Max Mushrooms, go back to the Master Dojo and talk to the guy who makes Max Soup, then give it to Urshifu. This will allow Urshifu to G-max.
Hope I helped!