Scythekicks hatch from eggs and grow into birds. We can technically be any kind of bird we wish even though the fan mascots are usually drawn as little yellow chicks. [Ref] Doppio often describes Scythekicks as delicious chicken, too, and likes to refer to any round, yellow objects as Scythekicks.
As part of the welcoming gift from the Headmaster and NIJISANJI, Doppio received the Arcade and the many arcade machines in it. One of the arcade machines is a UFO catcher (aka. crane game machine) filled with decorated eggs as prizes. Every time Doppio inserts a coin and earn an egg, that egg hatches into a Scythekick. [Ref]
Another explanation is that when Doppio starts streaming, people who are in the Arcade turn into little chicks — Scythekicks.
Even though we have an origin lore from Doppio, he still encourages people to create their own Scythekick sonas and individual lore.
Doppio is well-known for his passion and effort in producing quality voice packs, with interconnected stories across multiple VPs as well as different timelines/AUs. You can listen to many different sides of Doppio in the VPs, and even encounter the Pioverse characters! Without spoiling the contents, here is "a guide to doppio's voice packs" made by @scytheclips_!
*Note: The Carrd website is not managed by the same project team and may be outdated.