The answers above don't work for mongodb 6. My workaround ;

import { ObjectId } from "mongodb";

const ids = ["6786b6789020e854f0099c0a", "6786b6789020e854f0099c0b"]
const userList = await db.collection("user")
    .find({ _id: { $in: ids.map(_id => ObjectId.createFromHexString(_id)) } })
    .toArray();