Java/Android is basically gibberish to me as is - Maybe it is obfuscated sometimes. But strings themselves (API endpoints) have to be present some way or another.

There is also something called "smali" which is an intermediate representation - You can edit it following something like https://stackoverflow.com/questions/31101740/method-to-add-a....

Finally, there are occasionally native ARM libs included in the jar - You can get these with tool "retdec" or just run strings on it to find interesting stuff