What is it specifically that you want the ap to do?
I don’t use apps because I pretty much know how carbs affect me and I can dose by looking at food. I’m also pretty good at guessing when it’s something new.
I mean I can count up the carbs like ok there are 60 carbs here, that’s going to be 4 units of insulin because I use a15:1 ratio.
I agree with @Timothy, and have been Type 1 for almost 60 years. So it’s just routine to do what has worked before. Having cgms also help to see fast rising or dropping trends and react quickly, especially with newer pump which can adjust insulin.
If you are new to diabetes, logs can help if you review them or use to get advice from Endo.
If you use pump, history data can be reviewed online. With Tandem X2, can be viewed in Phone app with both cgm and pump data. Not sure about Medt or Omnipod but likely have similar.
For reference setting, I have a Dexcom 6 and use pens for insulin. My A1c is good but not great, meaning under 7.0 but not under 6.0, and about 70% time-in-range (TIR). If you want even tighter control, you might want others’ advice, but here is mine.
Dexcom 6 app for direct information on my Samsung phone
Dexcom Clarity for keeping an eye on my trends
A Samsung Pro 5 watch (my mg/dl appears as a complication)
Two additional apps to be able to see my numbers on the watch (linked below)
Seeing my numbers on my watch is essential, and during a transitional period when I bought the watch and was unable to see my numbers, my blood sugar control suffered.
As for building your own app, I have sometimes considered this as well. I work as a software engineer and have sometimes toyed with the idea of building one compatible with Google Assistant to use with my smart home devices.
You might have better luck with this kind of question in the tech forum. It is likely that the idea has been discussed thoroughly: