This is a commander cube.
The only notable changes from traditional commander rules are a change in deck size, and that any card can be put into your deck, regardless of your commander's color identity. However, you may only ever produce mana that is in your commander's color identiy (companions do not contribute to color identity). The upshot of this is that you can play cards with hybrid costs or with costs in the text box (though you will likely be unable to pay those costs). E.g. A player who has Arahbo as their commander could use Lurrus in the deck or as a companion.