After passing through Route 6, you go east to Royal Avenue, home of the Battle Royal Dome.
Explore Royal Avenue
Check to the right of the Pokémon Center building to find a sparkle to put into the Zygarde Cube.
There is a hidden Rare Candy to the right of the Malasada Shop.
There is a Poké Finder spot at the circle of flowers between the Pokémon Center and the Malasada Shop.
In the northwest part of Royal Avenue, there is an Ultra Ball.
Go to the Battle Royal Dome and you will see Gladion go inside.
If you go into the Thrifty Megamart, the guy at the entrance gives you a coupon. You can use it to buy supplies for less than the usual price if you talk to the checkout clerks.
Talk to the nearby clerk to get a free berry.
Talk to the Drifloon in the upper-right part of the store to get a Lemonade.
Check behind the Thrifty Megamart to get an HP Up.
Check in the Thrifty Megamart parking lot to get a green sparkle for the Zygarde Cube.
Battle Royal Dome
If you are ready, go into the Battle Royal Dome and approach the front counter. The Masked Royal, who might or might not be Professor Kukui, explains that in a Battle Royal, four trainers each pick three pokémon and battle each other. When a trainer's three pokémon all faint, the battle ends. The winner will be the one who defeated the most pokémon and has the most left.
You will have a Battle Royal now, against the Masked Royal, Hau, and Gladion. For this battle, there will only be one pokémon per trainer.
After the battle, Kiawe will introduce himself. He says to go up Route 7 to reach Wela Volcano Park.
You can go to the front desk of the Battle Royal Dome to start a Battle Royal or trade your BP for prizes.
Otherwise, if you are ready, put Water pokémon into your party and go northeast in Royal Avenue to reach Route 7.