I don't understand why there's no official phone app for Arduino.
The only app that is able to modify and upload sketches to arduino using the USB plug is named Bluino Loader, it has crash issues and the programmer didn't work on it for years.
Maybe I am missing something ?
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I would agree with S2k here.
There are too many variables involved with phones to be able to supply a single multi platform app.
If it is Android there are a few others available but we only ever see very few Q. regarding phone apps.
Memory requirements may also play a large role in making phones difficult to code for.
Some require an OTG adaptor to even start working.
Most of the MAJOR platforms are already covered by Arduino and the teams are slightly stretched to cover all those and new development.
There isn't an official app, but Arduinodroid has been available for a while and has over 1/2 million installs. I wouldn't suggest using a phone though, unless you have a BlueTooth keyboard. Fine on a tablet, with a keyboard: ArduinoDroid - An Arduino IDE for Android