"Pokemon Sun and Moon" has Alola region featured in it, the moment the game was released and played by various, hungry players and fans from around the world. The game features a whole new world including new generation Pokemons that will have a unique attacks including items and Z-moves. In addition, new generation Pokemons are not only present in the said game, but they also have legendary Pokemons and Ultra beasts because a Pokemon game cannot be completed without signature Pokemons that represents that version, which is "Pokemon Sun and Moon".
Encountering and capturing Solgaleo or Lunala
In either versions of "Pokemon Sun and Moon", Solgaleo can be found on Pokemon Sun version while Lunala can be found on Pokemon Moon. However, it is not that difficult to find these legendary Pokemons because you just have to progress in the game and can find the Pokemon Cosmog in the Altar of the Sunne or Moone, which are located in Poni Island. Once you progress the game and reach to the point where you are in either of the two altars mentioned (depending on the version you are playing), you find Cosmog, which evolves into Solgaleo or Lunala.
Zygarde
As you progress through "Pokemon Sun and Moon", you encounter the characters from Pokemon X and Y, which is Dexio and Sina and gives you a Zygarde Cube. The main thing you just have to do is collect Z-cells and cores located everywhere on the islands of the Alola region. Once you have collected enough such as 10 pieces, 50 pieces, 100 pieces, you directly go to the Route 16 on Ula-ula island and have it assembled. However, take note that you can get varied results, depending how many Z-cells and cores you have collected.
Other legendary Pokemons and ultra beasts
You simply have to progress and finish the game because everything will fall to place after that. For Ultra Beasts, an International Police will go to you and say something about these beasts and requests to capture it. Lastly, as for the guardians of the Alola islands, when you become the Pokemon Champion of Alola, these guardians or Tapu guardians appear in their respective caves and ruins. Other Pokemons such as Magearna, Marshadow are two Pokemons that will still be unlocked in the future and creators will still release the QR codes to be scanned in order for these Pokemons to be unlocked in "Pokemon Sun and Moon". More detailed guides can be viewed on BGR website.