Pokémon games had a system where you had to assign specific abilities to travel across the world and traverse the environment. For example, you needed a Pokémon capable of knowing the move ‘Surf’ to travel across water. Things changed in Pokémon Let’s Go Pikachu and Let’s Go Eevee, and it made things much easier. The same goes for the ability to fly from town to town.
If you want to fly in any of the Pokémon Let’s Go games, you need to use the sky dash ability. You discover the ability by naturally progressing through the game, and after the third Pokémon Gym, and after you fought Giovanni at the underground Team Rocket base. An NPC should show up who is standing next to a bicycle with balloons attached to it.
You should see the individual next to the floating bicycle outside of the Gaming Corner. Approach him and ask him about his flying machine. After you receive the device from the man, all you have to do is open up your Play With Pikachu or Eevee screen to access the secret technique. You can now travel instantly to any town you have visited moving forward, making it significantly easier to go from place to place.
Previously, players had to give specific Pokémon the useless moves, and they were permanent. Newer Pokémon games have steadily been removing them, such as in Pokémon Sword and Shield, where players relied on a Rotom Bicycle to swim across waters. The changes have been extremely welcome.