How do I create a repeating timer on iPhone?

A timer that restarts would be useful, is there a way to do it on iPhone and iPad?

Yes and no... iOS 11 allows you to quickly repeat a timer but doesn't let you set it to automatically repeat. If your timer ends while your lock screen is on you will see a button to repeat it. If you aren't on your lock screen you will need to deep press the clock notification for the repeat button to come up.

How to create a repeating timer on iPhone and iPad.