Create an live segment app for garmin.

Ciao ​@Tobiasz Marszalek. I don't know if I correctly get your request.. do you want Strava segments to be shown on Garmin? If so it's already possible

How? I’m thinking about live segment on watch.

Live segments are already available with at least some Garmin watches, no special app needed.  For instance, I just downloaded the manual for the Fenix E, did a search on the word "Strava", and it shows you how to do it.  I don't know if that's true for ALL Garmin watches... a few of their lower priced bike computers don't have segments.  But most do.  Look at that owner's manual!