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In fact: duplicating
When anything like 2>&1 is encountered, it means that anything that descriptor 2 contains is copied to descriptor 1, thus the order of "redirects" matter:

ls / not_existing_file 2>&1 1>/dev/null - the stderr is copied to whatever the descriptor 1 points to (stdout by default), the stdout is copied to /dev/null, the stderr is not updated

ls / not_existing_file 1>/dev/null 2>&1 - the stdout is copied to /dev/null the stderr is copied to whatever the descriptor 1 points to (dev/null in this case)

Closing file descriptors

To close descriptor, duplicate -, e.g.:
Close stdin: 0<&-
Close stdout: 1<&-

References

  1. https://wiki.bash-hackers.org/howto/redirection_tutorial