@New-Song
As a pilot, I use ForeFlight and this is my procedure to do everything on my iPhone:
FOREFLIGHT FPL-KML-GPX
ForeFlight
Flights, Send To, Map
In Map, Send To, SHARE FPL FILE, choose location
https://www.gpsvisualizer.com
Upload a GPS file:
Choose an output format: GPX
WINDY
Logo account
Scroll down, more, more, ….
Route saved (choose any)
…More
Import GPX
Choose GPX file