When you convert a YouTube video file to an MP3 audio file, you're essentially extracting the audio from the video and saving it as a separate audio file. Here's a general overview of how this conversion process works:

1. Find the YouTube video you want to convert: First, you need to locate the YouTube video you want to convert. You can do this by searching for it on YouTube or by copying and pasting the video's URL.
2. Use a YouTube to MP3 converter: There are a variety of online tools and software programs that allow you to convert YouTube videos to MP3 files. Some popular examples include to-convert or others similar. These tools typically work by pasting the video URL into a field on their website or software, selecting the desired output format (in this case, MP3), and then clicking a conversion button to start the process.
3. Extract the audio from the video: Once you've started the conversion process, the tool you're using will begin to extract the audio from the video. It does this by decoding the audio data within the video file and then re-encoding it in the MP3 format. The specific method for doing this may vary depending on the conversion tool you're using.
4. Download the MP3 file: After the conversion process is complete, you'll typically be prompted to download the resulting MP3 file. You can then save this file to your computer or other device, and listen to it using a variety of software or hardware players that support the MP3 format.

It's important to note that while YouTube to MP3 converters are widely available and easy to use, they may not always be legal or ethical to use, as they often violate YouTube's terms of service and can be used to download copyrighted content without permission. Additionally, some conversion tools may contain malware or other security risks, so it's important to use caution and only download files from reputable sources.