FFMPEG from Zero to Hero
Might have come in handy while I was struggling to type out    ffmpeg -i part0.mp4 -i part1.mp4 -filter_complex "[1:v]scale=1280:720:force_original_aspect_ratio=decrease,pad=1280:720:(ow-iw)/2:(oh-ih)/2[v1]; [0:v] [0:a] [v1] [1:a] concat=n=2:v=1:a=1 [v] [a]" -map "[v]" -map "[a]" out.mp4

to concatenate two videos with different sizes today.My relation with it is almost identical to my relation with any bash scripting more than a one-liner: I have to relearn it every time I want to engage with it.