There is nothing to explain really. You just open the video with your favorite audio or video editor as most support opening the most common video formats and just extract the audio. How to do that depends on the application that you are using and details can be found by googling about the Application you are using and where to find the export or save it as audio.
For filmora for example you just select the export video and then from the filetypes, mp3.
For Premiere Pro, select one or more clips containing audio, choose Clip › Audio Options › Extract Audio.
VLC, pretty much the same, you can select save/convert from the menu, add the files you want to convert, and choose an audio format to export.
For more details it's best to search for one of the many online guides for the apps you use, cause as you can understand, it impossible to cover all of them.
