convert webm to mp4

It will convert to 128kb/s

ffmpeg -i input.webm -c:v libx264 -preset slow -crf 22 -c:a aac -b:a 128k output.mp4

remove from the middle

Method1 (worked!)

Ref

# will remove the 121 to 190 seconds from the full video
ffmpeg -i input.mp4 \
  -vf  "select='not(between(t,121,190))',  setpts=N/FRAME_RATE/TB" \
  -af "aselect='not(between(t,121,190))', asetpts=N/SR/TB" \
  output.mp4

Method 2 (not tested)

ffmpeg -i input.mp4 -t "$start_cut_section" -c copy part1.mp4&
ffmpeg -i input.mp4 -ss "$end_cut_section" -c copy part2.mp4&
echo "file 'part1.mp4'" > filelist;
echo "file 'part2.mp4'" >> filelist;
wait;

ffmpeg -f concat -i filelist -c copy output.mp4;
rm filelist;

or Ref

ffmpeg  -t 00:11:00 -i input.mp4 -map 0 -c copy segment1.mp4
ffmpeg -ss 00:11:10 -i input.mp4 -map 0 -c copy segment2.mp4

# put these in input.txt
file 'segment1.mp4'
file 'segment2.mp4'

ffmpeg -f concat -i input.txt -map 0 -c copy output.mp4

trim (cut)

# time-based
ffmpeg -i input.mp4 -ss 00:02:01 -to 00:03:08 -c:v copy -c:a copy output.mp4

# duration-based
ffmpeg -i input.mp4 -ss 00:02:01 -t 67 -c:v copy -c:a copy output.mp4