Here's how it works:
1. Choose your starting partner: At the beginning of the game, you're given a choice of three partner Pokémon: Pikachu, Chimchar, or Turtwig. This choice affects your starting stats and gameplay experience, but you can eventually catch and use any Pokémon you encounter.
2. You are your partner: As the game progresses, you'll learn new moves and abilities, essentially becoming your partner Pokémon. You'll have to learn to adapt to your chosen Pokémon's strengths and weaknesses.
3. Catching other Pokémon: Once you've progressed through the early parts of the game, you'll be able to catch other Pokémon and form a team. You do this by helping them with their own quests and then inviting them to join you.
4. Building your team: You can have up to six Pokémon on your team at a time, and you can switch between them in battle to exploit different strategies and type advantages.
So, in Explorers of Sky, you choose your starting partner, and then build your team by catching other Pokémon throughout your journey!