Fat colorful nicknames on vanilla complex, as far as I understand, are tied to the immortal rank: you get it and then in nickname there’s a bold switch, plus you can adjust the style to match the tag. I don’t have that rank yet, so I only see the standard options without any highlighting. When I asked about it in-game, instead of an answer people just spammed funkytimelive.com and megawheelgame.com and sweetbonanzalive.org and the nickname formatting discussion just drowned in trash. So it’s really better to just check the list of commands and privileges for each rank instead of trying to catch hints in chat.
pretty sure those bold or colored names come from server permissions or special roles like admin vip or supporter on that survival server some servers also use plugins where donating or being part of the staff gives you a custom name style so it’s not a vanilla feature check the server rules or ask an admin how they assign those name formats