[FFmpeg-user] Assembling files from several input files

[Werner Robitza]

Let's assume I have four files:

video1.mp4
video2.mp4
audio1.m4a
audio2.m4a

What would a simple command be that could concatenate/mux the files in such
a way that it contains:

video1 from 0 to 20 seconds, video 2 from 20 to 30 seconds
audio1 from 0 to 10 seconds, audio2 from 10 to 30

Basically, I need to freely mux/concatenate parts of different sources. I
know I could cut the video parts individually before, then concat them into
a final video, then do the same with audio, and finally encode and mux
everything, but would it be possible to do that in one filter expression?