>>9297816MyGM is more "gamey". You're playing against AI or another player(s), you have goals related to ratings and money, there's less freedom/customization (e.g. you can only choose from a few different brands, your roster is limited by the draft/money, your arenas are based on how much money you have etc.)
Universe mode is like a sandbox that you can mold to your liking. You can freely decide how many main shows, B shows and PPVs you do, which wrestlers are on which roster and division, which titles/arenas/logos etc. are used, who wrestles who, you can decide everything and don't have to worry about doing something wrong and "losing". Some people just try to replicate current day WWE and book it how they like, some create their own fictional promotions and fill them with fictional wrestlers, some do something in-between. You could do WWE vs AEW and have the wrestlers jump from one company to another on a whim.