Seemingly overnight, everyone’s talking about Temu, an online shopping app full of deals that seem too good to be true. You’ll find $17 wireless earbuds, $1 “gold” necklaces, and $23 wedding dresses.

You can already buy almost anything with the click of a mouse. Now tech giants are preparing for an AI helper to do even that for you one day. Google, Amazon and OpenAI are racing to add new ...