We use Android devices for any portable interfaces in the field, along with an FTDI cable attached to the 6-pin FTDI header of the Mayfly. The Mayfly v1.1 has it’s own CP2102 USB chip on board that works through the USB-C jack on the Mayfly, but Android devices can’t natively use it because they don’t recognize the CP2102. Plus with the inconvenient angle of the Mayfly’s USB jack anyway, it’s just more convenient to use a standard FTDI cable on the Mayfly’s FTDI header. Just have to be sure to get the pin orientation correct! Previous Mayfly versions (v0.5b and earlier) had an FT232 chip onboard, so it was possible to connect an Android device directly to the Mayfly through the micro-USB jack (using an OTG adapter on the cable too), but again the mounting of the USB jack isn’t convenient for access when a logger is installed in our usual enclosures.
Since I’m not familiar with iOS devices, all I know is that the last time I researched the issue, iOS doesn’t officially allow direct connection of serial (or TTL) accessories to Apple devices through the lightning port, plus there are no official apps in the App store even if you could buy a cable. There are some unofficial interface cables that some companies have made that may or may not work with a device like the Mayfly, and you supposedly would then have to use software to create your own app to actually handle the data. So in the end, everyone who has asked me about this issue with a Mayfly ended up just using an Android device and an FTDI cable. (just be sure the FTDI cable or board is based on the FT232 chip and not a CP2102, for the reason mentioned above).
Since reading live data in the field directly on the Mayfly board can be very helpful sometimes, I developed this small OLED display a couple years ago: https://www.envirodiy.org/product/envirodiy-oled-half-shield-pack-of-5/ It works great with basic sketches, and can be switched on and off in the code to save power (like having the Mayfly’s onboard light sensor automatically sense the light levels and only activate the display when the enclosure lid is open, or when someone presses a button). We’re still working to incorporate it into the existing ModularSensors logging sketches that are usually used in our full deployments, but it could eliminate the need for phone interfaces for some users.