Take an example.

For command:

ffmpeg -i 1.mp4 -i 2.mp4 -i 3.mp4 -filter_complex "[0:v][0:a][1:v][1:a][2:v][2:a]concat=n=3:v=1:a=1[vout][aout]" -map "[vout]" -map "[aout]" output.mp4

It can be visualize as:
enter image description here

You can visualize your ffmpeg command via https://ffmpeg-graph.site/