When you cast the card, it'll enter the battlefield under your control
Each have their own strengths and weaknesses, and I think there's a decent argument for having any of them as your commander: Szarekh, the Silent King is the face commander of the precon and gets a bad rep from the community for being "bad" when in reality it's the best commander for the stock list
Some of the best cards to pair with Kenrith, the Returned King include: Training Grounds will dramatically reduce activation costs for Kenrith Biomancer's Familiar provides another source of activated ability cost reduction Agatha of the Vile Cauldron can actually drive all of Kenrith's abilities to just a single mana cost Illusionist's Bracers copies Kenrith's powerful activated abilities 3
Hugs, Taxes, and Aggro View on Archidekt Commander (1) 1 Ms
This means you could drop a land and a few Mox, then trade your hand in for a brand new one on your first turn, and leaving your opponent with a potentially bad hand.