After you learn Rock Smash, you can unlock Palette Town, an area where you can customize at your own pace and play with other players online.
How to Unlock Palette Town
In Withered Wasteland, build an Exercise Resting Spot habitat (place a bench next to a Punching Bag) to make Hitmonchan move in. Then talk to Hitmonchan and give them two stones. You will learn Rock Smash.
With this, you can go to the western part of the area and cross the bridge to reach Palette Town.
Check the PC
Go to the Palette Town Pokémon Center and check the PC. You can give Palette Town a unique name, and you can always rename it by pressing + while using the Palette Town PC.
You will see that there are special challenges at this PC that reward you with special items. These items can be used to create unique habitats in Palette Town.
Meet Eevee
After this, Eevee will greet you. You will learn all the habitats that will bring back Eevee's friends. You can get the items for these habitats by completing the special challenges at the Palette Town PC.
Get Special Building Kits
On the three islands in Palette Town, you will find yellow Poké Balls on top of each island. Each Pokéball contains a special building kit. Build these in Palette Town to summon Articuno, Zapdos, and Moltres.