You need a "USB OTG adaptor" in either USB micro-B or USB-C flavour as appropriate to the socket on your phone (don't use one in combination with micro-B to C adaptors, that doesn't work). This should give you a USB-A socket (like you'd have on a computer). Plug your regular USB A to USB mini-B cable into that. Wait an implausibly long time while the eTrex boots up and becomes a USB storage device. Wait a little longer for the Android to recognise the device and mount it. (You should get a notification, similar to the one you get for an SD card when you first boot up.)
Some old Android versions get confused by the eTrex appearing as two USB-storage drives (one for the internal storage, one for the SD card) and only mount one of them. If it only sees the card, you can make it see the internal storage by removing the SD card from the eTrex before plugging in.)
Then you just need some sort of file manager app to copy your GPX file from the phone's local storage to the eTrex. I don't use RwGPS, so can't comment on whether their app is able to write to a USB-storage drive directly (this is one of those things that's changed several times between Android versions, so older apps sometimes don't know that they need to ask permission to access it and fail with an unhelpful error).