Stream is the base class for character and binary based streams. It is not called directly, but invoked whenever you use a function that relies on it.
Stream defines the reading functions in Arduino. When using any core functionality that uses a read() or similar method, you can safely assume it calls on the Stream class. For functions like print() , Stream inherits from the Print class.
Some of the libraries that rely on Stream include:
available()
read()
flush()
find()
findUntil()
peek()
readBytes()
readBytesUntil()
readString()
readStringUntil()
parseInt()
parseFloat()
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