One easy to remember way to open tracks in an app is to email the file to yourself as an attachment. Use a good subject title. When you need the track you open the email and then the attachment. Your phone will see the gpx, kml or kmz suffix on the filename, look up what apps you have that registered themselves as being able to handle files of that type and then ask you which of them you want to use.
There are other simple ways to open the tracks with an app but it depends and the app you want or file manager that you use on your smartphone so it is more difficult to EXPLAIN how to open the track those ways.