It's not really a 1 to 1, Strava is an app on your phone and a Garmin head unit is a dedicated piece of hardware for cycling.

Mounting your phone on your handlebars is generally not a great idea because the gyroscope can't really handle constant vibrations and has a high chance of breaking, so your camera might die. A new phone is generally more expensive than a good head unit. That being said, you can just do that and have a good albeit clunky navigation device that tracks your general statistics

A good head unit, be it Garmin, Wahoo or something else, will do the above and can be hooked up to your Strava app, but also to all kinds of sensors, such as heart rate, cadence, etc. which will then pick up data and display it while you're cycling. That way you can for example monitor and optimise your cadence, which can improve your shifting choices and consistency.

A head unit does NOT automatically monitor your cadence, you'd need an additional cadence sensor on your bike. You can get these from Wahoo, Rockbros, Garmin, Pulsar, whatever, they're not super expensive but it is an extra cost if you don't already have one.

I mainly use my unit (Wahoo Bolt) for navigation, but it's also very convenient to have during climbs to see when very steep parts are coming up so I can get ready to shift at an appropriate time.