How to combine audio and video with ffmpeg

When you downloaded them separately

Sometimes, download managers offer two options when downloading videos from a video-hosting websites: either you can download high-resolution video, or a video with audio. But what if you want both: high-resolution video together with audio? Then you can combine them!

Imagine that files you downloaded are named audio.mp4 and video.mp4 (note that "audio" file does include video, too, but we're not interested it it). Then you can just run a simple command like this:

ffmpeg -i video.mp4 -i audio.mp4 -map 0:v -map 1:a -c copy -shortest output.mp4

Thanks to this stackoverflow answer for a detailed writeup!