Last time I had anything to do with Arduino was quite a while ago… This time round, I thought I should be able to do it on my phone instead of Windows, but it looks like that was a temporary fad, which is a shame… about the only IDE I could find in the Play Store was Arduino Studio, which seemed decent enough but turned out to have broken OTG support, with no way to grant permission to access USB.
ArduinoDroid appeared to be the other main (or perhaps only) contender, but it’s no longer under active development, so Play Store doesn’t allow the option to install it on current versions of Android. I had a dig around, and found this page which hosts a bunch of versions including the latest one from July '22: https://arduinodroid.en.uptodown.com/android
If you don’t know how to get an APK onto your phone, just search “sideload APK”. I tried it out and it worked great; seems like a pretty comprehensively sorted bit of software, and was easier to get going with than Android Studio, with about 60% the installed size at 600-odd MB, including a bunch of libraries and example sketches. I was really getting into it once I plugged in a USB host with keyboard and mouse in addition to the FTDI cable, when it started spamming me with ads.
I was a bit dubious about paying the ten bucks to unlock it, given the payment was processed through Google and I’d sideloaded it, but given I was pretty stoked with it until the ads and there were no other options, I coughed up. Was briefly cheesed off when the ads didn’t stop, but had a look at the support page, which said to restart the app, and bingo. Pretty happy with that purchase.