Export Audio to MP4 creates a separate file instead of exporting to source video

Windows 10, version 21H1, OS build 19043,1415
Audacity 2.4.2
I’ve been using Audacity for audio work for a decade, but I’m new to videos.
Situation: start with a video, ChrIsnCom.MP4, abt 400 MB.
I downloaded and installed FFMPEG a couple of months ago.
Import Audio, browse to my MP4. That part worked perfectly.
Edited the audio file.
Export Audio, select Custom
Click the button to select custom format and codec.
Select MP4 and the first of the three codecs.
(There are a lot of fields to fill in. I leave them alone, assuming that the defaults will work)
Click OK. Audacity says it is exporting the audio.
Look at the files in the directory. There is a new ChrIsnCom file of type “File”, according to Windows. It’s about
1 MB, so quite possibly it’s the audio track
Play ChrIsnCom.MP4. The audio has not been changed.

I hope this is an easy one, but I can’t find anything applicable in the form. Thanks!//Zeke Hoskin

When you open an audio/video file in Audacity the video is ignored so all you have is the audio. And if you export you’ll get an audio-only file.

You’ll have to use a video editor (or some kind of audio/video tool) to combine the new audio and old video.

Audacity doesn’t edit video. It will open the audio portion of many different video formats, but it won’t put the new audio back into the video file.

You need a video editor.