Welcome to the Ranch.
On Android (as well as iOS) you generally don't use save buttons - data is saved automatically, usually from the Activity's onPause handler.
There are various ways to store data persistently; which is the most appropriate one depends on the amount of data. For key/value data,
shared preferences might work. For regular data, possibly the built-in SQLite database via the
Room library. Even
files are a possibility, although I would advise against those.