ffmpeg

Trimming an avi file:

ffmpeg -i Die_Sendung_mit_der_Maus_16.06.19_09-30_ard_30_TVOON_DE.mpg.HQ.avi -vcodec copy -acodec copy -ss 00:06:04 -t 00:29:32 output.avi

ss is the start time and t is the duration (not the end time).

webM to mp3 conversion:

ffmpeg -i La_robe_de_soie.webm -vn -ab 128k -ar 44100 -y "La_robe_de_soie.mp3"

mkv to mp4 conversion:

ffmpeg -i input.mkv -c copy output.mp4

Concatenate two videos:

ffmpeg -f concat -safe 0 -i list.txt -c copy /tmp/output.avi
cat list.txt:
file 'video1.avi'
file 'video2.avi'

For the more advanced functionality, the syntax becomes impossible to remember. Hence the need for a GUI that makes ffmpeg more accessible. The best tool that I have come across so far is the cross-platform LosslessCut  . There are also packages for Mac, Linux or Windows  .