Usage of ffmpeg complex filter

The goal: to pick two segments, 0-10sec and 20-31sec, from a 41 second long video in.mp4, to form a 20 second long video out.mp4.
The code:
ffmpeg.exe -i in.mp4 -filter_complex ^
“[0:v]trim=duration=10[V0]; [0:a]atrim=duration=10[A0]; ^
[0:v]trim=start=20:end=31,setpts=PTS-STARTPTS[V1]; [0:a]atrim=start=20:end=31,asetpts=PTS-STARTPTS[A1];[V0][A0][V1][A1]concat=n=2:v=1:a=1[V01][A01]” -map [V01] -map [A01] out.mp4
-i what’s following is input file.
-filter_complex complex because there are intermediate outputs.
[0:v] 0 means the input file after first -i, in.mp4 in this case. v means video stream.
[V0] the name of output of [0:v] after applying “trim=duration=10”. the first video output
[0:a] similar to [0:v] but audio.
[A0] similar to [V0] but audio. the first audio output
[V1] and [A1] are the output video and audio of second segment.
setpts and asetpts means to reset the Presentation Time Stamps.
-map tells what output to mux into the final output file.
out.mp4 is one of the output files, which is for the -maps preceding it.
what follows -map can be:
-map [V01] filter output name, like in the example above.
-map single_highest_quality_video_stream_from_all_inputs
-map 0 means the video and audio of the first input file

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