Standard Streams
These are the ways a process is allowed to interact with the external environment.
stdin - Defaults to keyboard stdout - Defaults the screen connected. stderr - Defaults the screen connected.
For new processes/commands, standard stream file descriptors are inherited in the new process. This is exactly why the command output is simply written to same place from where the command is executed.
Information in File Descriptors
Every process has a file descriptor for all IO as well. The first 3 file descriptors for any process are 0, 1 and 2 which is stdin, stdout and stderr respectively.
The file descriptors fo these standard streams are address to a buffer object (backed by arrays or linked lists) from which the data is read or written.
Standard Streams for GUI applications
The standard streams exists for all process - Terminals as well as GUI/desktop applications.
In desktop applications, the standard streams are sometimes connected to '/dev/null'.
Stdout of a UI application isn't what we see on the UI.
Stdout here is used to write some debug logs to log file or send some data to some network socket.