How do stdin/stdout and file I/O work at a high level in any language?

stdin, stdout, and files

Programs communicate through streams: bytes flow from input sources (keyboard, file, socket) to your code and from your code to outputs (console, file, HTTP response). The OS buffers I/O and exposes a small API—read/write or higher-level scanners.

• stdin (standard input): Default input stream—keyboard in terminal apps.
• stdout / stderr: Normal output vs error channel (logs still visible when stdout is piped).
• File I/O: Open path → read/write bytes or lines → close (or use try-with-resources).
• Encoding: Text files are bytes + charset (UTF-8)—always specify encoding in production.

try (BufferedReader br = new BufferedReader(new FileReader("data.txt"))) {
  String line;
  while ((line = br.readLine()) != null) System.out.println(line);
}