If you need to add cover art to a podcast for upload as a video to YouTube:
ffmpeg -loop 1 -r 1 -i hospitals.png -i 20210917_Hospitals.mp3 -c:a copy -shortest -c:v libx264 20210917_Hospitals.mp4
If you need to recombine audio you extracted from the same video, because you needed to edit the audio separately in audacity:
ffmpeg -i HDV_1167.MP4 -i HDV_1167.mp3 -c:v copy -c:a aac -map 0:v:0 -map 1:a:0 20210525_MOAR_CRAZY.mp4
combine several videos together:
cat mylist.txt file '/home/doctor/Desktop/HDV_1162.MP4' file '/home/doctor/Desktop/HDV_1163.MP4' ffmpeg -f concat -safe 0 -i mylist.txt -c copy troll_s1.mp4