Email your (or any) gpx to yourself, airdrop whatever, and with Garmin Connect on the phone the gpx will open it and display details and map, tell it what type of route it is road/mtb/whatever and then using the 3 dots/slashes 'send to device', garmin connect will prepare it for transfer to the Explore when they next synchronise.
Sometimes I have to rename the import from 'transfer date' to whatever, and note that renaming on a phone is easier than on GPS, I can't actually remember which GPX source has that drawback.
Cycle.travel linked to your garmin account allows 'send to garmin connect' and is an unbeatable combination.
I'm poor (and like to meddle...) so this is using W10 and android.