what is the meaning and usage of 2>&1 | tee /dev/stderr

for the following command output=$(cat $file | docker exec -i CONTAINER COMMAND 2>&1 | tee /dev/stderr) what is the meaning and usage of 2>&1 | tee /dev/stderr? I google and 2>&1 means "combine stderr and stdout into the stdout stream." tee mean "reads from standard input and writes to standard output and one or more files simultaneously." but I cannot understand if combine 2>&1 | tee /dev/stderr 1) this combine stderr and stdout into the stdout stream, but then again write it to /dev/stderr (&2) again? 2) why need to combine to stdout, then redirect back to stderr? 3) and after write to /dev/stderr, will the message save to variable output in the following? output=$(cat $file | docker exec -i CONTAINER COMMAND 2>&1 | tee /dev/stderr)