"Utopia", known as "King of Wishes, Hope" (希望皇ホープ Kibō'ō Hōpu) in Japan, is an archetype associated with the "Number" archetype. It is a sub-archetype to the "Hope" archetype. They are used by Yuma Tsukumo and Astral as their signature cards in the anime, and are supported by cards in the "ZW -" archetype.
"Utopian Aura" and "Utopia Buster" are not "Utopia" cards because they do not have 「希望皇ホープ」 in their Japanese names.
Aside from "Number 39: Utopia" itself, the monsters have effects which either increase their own ATK or deal with the ATK of the opponent's monsters.
A Utopia Deck usually forces on Xyz Summon Utopia monster and either Negating Monster Attack or Monster effects and the Number C39,Rank-Down, and Rank-Up version focus on Gaining ATK and opponent's monster losing ATK. Utopia's alternate from can be easy summoned by most Rank-Up-Magic cards and Rank-Down-Magic Numeron Fall. This archetype can work well with Number Wall and ZW - due to the archetype being a Number and Utopia monster. Another Great card that can work with Utopia is the Field Spell Card, Onomatopia which supports this archetype along with other archetypes.
Two great Xyz Monsters that can support this archetype are Number 99: Utopic Dragon and CXyz Barian Hope. Number 99 can be easily summoned to the field by simply discarding a Rank-Up-Magic card from your hand and using a Utopia as a Xyz Material. Barian Hope can be Xyz Summon by Rank-Up-Magic Astral Force, giving that monster ATK from Xyz Material and using the effect of Utopia monsters in the Graveyard.