Your smartphone can be attached to the handlebar thanks to a smartphone bike mount. A lot of different brands are selling such mounts. We personally love the ones from the brand Quad Lock.
The model of case you need to choose depends on your smartphone, this is why we don't include one by default with the bike.
Such a mount is not required to be able to use the bike though. Once the bike is on, you can simply leave your phone in your bag and ride.