I got Solid Explorer for our Moto, didn't like it and then found it was not needed as there was a simple file manager in Android.
With the Moto G5 Plus I do as follows:
Connect up the OTG cable set and wait quite a while for the etrex to display the USB symbol to show it is in mass storage mode (I have it set to default to mass storage - you can have it set to prompt).
Swipe down on the Moto and see Android System V Garmin USB drive listed (or drives if you have an SD card in place).
Touch were it gives the drive and you go into a simple file manager looking at the etrex internal or SD memory but you can now escape to look around the Android file system.
Touch the three bars on top left and it moves up the combined file system showing the Moto's internal memory as well as Downloads and the Garmin drives.
Navigate around the Android memory to find the file to transfer and select it with a long press - shows selection with a tick.
Open three dots menu at top right and select Move to..
Use the three bars menu to go up the file system and select the target Garmin>gpx on the Garmin internal or SD card and confirm the move.
Unmount the Garmin drives using the underlined triangle and then unplug the cable. Route or track should now be available on the etrex
I don't know how Android generic this is or how to get into this file manager in other contexts. The file manager lives on after you unplug the OTG. I will be interested to know if this works for you...
