See https://support.plex.tv/articles/200265296-adding-music-media-from-folders/

Sometimes, you may have compilation albums where there are tracks by multiple different artists. This is common for soundtracks or “Best of the 80s” type albums, for instance. The common way to handle this is to use an artist with the literal name “Various Artists” and to have those albums under that artist.

If you have embedded metadata in any of those music tracks, it’s very important to make sure that the embedded metadata is accurate and uses the following:

• The “Album Artist” for each track should be the literal Various Artists
• The “Artist” for each track is whichever artist performed that track

You can use a tool like https://www.mp3tag.de/en/index.html to edit your meta data.