Spent days to setup a newer-Android version with reverse-proxy-HTTPS certificate in its CA store + one that'd support Google Play and signing in (old school man-in-the-middle).
FINALLY got the API calls of this 1 app whose unofficial client I wanted to make coz their main sucks ass. Just to get stuck on the phone-number-based OTP that they use for their login (:

They send a unique token for each OTP request, I assumed they're using some hard-coded string based function, which they decrypt on their backend to verify.
Downloaded their APK and decompiled. Went through dozens of weird-ass-named classes (coz decompiled). For the 2nd time I thought I had it!

But no -.- they call Google's Firebase messaging for the phone-num OTP n that function simply called firebase, looked into that service n ofc it's very tightly coupled with the calling API's backend

It was fun while it lasted I guess~~~