Howdy there - the field Plex uses for “primary” artist matching is “Album Artist” (the tag is titled “albumartist”).
The “Artist” field on the other hand can be just about anything - Plex won’t throw a fit (generally) when matching based on this one. This is where I keep featured artist credits.
Just note that Plex doesn’t handle additional artist credits very well overall, but there’s even more strange behavior if they are not separated by a semi-colon.
I also use a script to replace “feat.”, “featuring” and “with” with a semi-colon:
With all that said, sometimes two artists are credited at the release level, and you just have to make a personal decision on who you want Plex to identify as the primary artist by whittling the “albumartist” tag down to one artist.