If a Link Monster points towards the Extra Monster Zone, and it is Co-Linked with another Link Monsters, which is Co-linked with another and so on, in such a way that it connects to the Link Monster in the other Extra Monster Zone, you can Summon a Monster the first Zone, thus creating an Extra Link
Typically these will be in your sideboard, but depending on the meta, your deck, and what the card does, sometimes you can be extra hateful and make it work mainboard
The six colors of Starter Decks are from previous series
anime and the decks it spawned, its a series notable more for its nostalgic, memorable storylines and many iconic cards and moments, rather than spawning new archetypes that would come to define this early era of the Yu-Gi-Oh
Mirror Universe Magus of the Mirrors namesake is Mirror Universe, a very rare Legends card that does everything the Magus does but faster and without the trappings of a creatures body