Bike computer which connects to iPhone?

There are two ways to do this, either using a device with GPS which connects to an app on your phone via bluetooth or wifi at the end of the ride. In this case you can ride for as long as the battery on the bike computer, and your phone's battery is not used during the ride. E.g. Garmin Edge series or Wahoo.

The other way is to use the GPS from your phone connected using bluetooth to a computer. This gives a lower cost device.

E.g.,

https://www.cateye.com/intl/products/computers/CC-RD500B/

A full list of supported Strava devics is here

https://support.strava.com/hc/en-us/articles/223297187-How-to-get-your-Activities-to-Strava#devices

Very old GPS computers used to require you to use a USB cable to upload manually. Now they pretty much all have apps and the process is seamless. So be careful if buying a used GPS. I don't know about iPhone but whatever you buy you'd want to check that there is an iPhone app

E.g., this page shows the Wahoo Elemnt app on Iphone which automatically connects using the Wifi chip in the Wahoo elemnt computer to upload to Strava. https://support.strava.com/hc/en-us/articles/218366788-Wahoo-ELEMNT-and-Strava

Any modern GPS enabled bicycle computer will sync with Strava and other cycling-related apps, e.g. BikewithGPS, Mapmyride. Garmin is probably the market leader and makes computers at many price points, but Lezyne and Bryton make good budget friendly computers, Wahoo is easy to use, and there are likely others I can’t remember right now. The key point is that GPS computers will connect to Strava.

You do have to turn on the computer and press the start ride button, plus you have to stop the ride. You also have to set up the computer on a mobile device, and link it to your Strava or other account. From there, you should not have to touch your phone on a ride.

In theory, you can use the computer without Strava. You would be just using Garmin’s, Wahoo’s, or whoever else’s app. However, they may not integrate all the functionality of Strava.