Update on already made mobile application

Yes, absolutely, as long as the original developer followed best practices. Just like a writer should make a book understandable to its readers, a developer should make its code easily understandable to other developers. How? To start with, by giving descriptive names to the variables, methods and classes, by sticking to naming conventions (camel case, etc...) , by indenting the code properly and maybe skipping a line where necessary, and by leaving comments in the code if some part needs extra explanation. Also, organising the classes in sub packages goes a long way (you can read more about architectural patterns). You can also check out code samples from Google (since you mention Android) or from renowned/experienced developers to get a glimpse of what good code looks like. There are many books on the subject that will go more in depth to help you write good, clean code.

There's a joke that goes:

Always code as if the guy who ends up maintaining your code will be a violent psychopath who knows where you live.

Finally, while there might just be a single developer working on a small app, it is common to have a team of developers working on the same app in the professional world, so the code needs to be understood by all and might be amended by anyone where needed. As long as everyone follows best practices and codes with the others in mind, and not just for themselves, it ¡s perfectly feasible.

Hope that helps :)