It's great to hear that you are already making games. That's the best thing you can do to get on the road to making games like Mario Odyssey.
As Johnathan mentioned, most commercial games are built by teams of people. Teams consist of artists, game designers, programmers, and producers. Within each of those groups, you will often find individuals that specialize in sub-disciplines. For example, the Game Design group often contains level designers, system designers, narrative designers, etc. Art teams often have 3D modelers, animators, and texture artists. Programming teams may have AI programmers, server programmers, UI programmers, audio programmers, etc. The bigger the game, the larger the team, and the more specialists you will find.
But don't get discouraged! Each year, there are more and more resources online to support independent game-makers in their quest to build great games with small teams. The Unity and Unreal engines have Asset Stores where you can by art, sound, design tools, and even code libraries. That means you can make neat games without needing a full team. Don't have a 3D artist? Go to the asset store and buy the 3D art you need.
This system isn't perfect. It can be hard to find exactly the pieces you need, and even if you do, the styles don't always match. But if you look carefully, you can often find pieces that fit together reasonably well. You can also find courses at sites like Udemy to help you build the skills you need to make the pieces work better. For example, you might find some 3D models on the Unity asset store that *almost* work for you, but need some tweaking. If you watch a 3D modeling tutorial on Udemy, you can get the skills you need to tweak those models to fit you game.
Make no mistake: building a full game like Mario Odyssey is a HUGE task that takes big teams many years to finish. But you and a friend or two could make a 3D platformer with a few good levels over the summer if you get good with one of the popular 3D game suites (like Unity) and buy some good models and animations from the asset store.
It all starts with learning 3D gaming -- and for that, I recommend you look into Unity. Unreal works, too, and is probably more powerful, but is harder to learn and doesn't have as mature an Asset store.
Whatever you do, keep making games! Don't underestimate how much you learn even making simple 2D games. But also don't be afraid to chase your dreams. If you want to make a 3D platformer, go for it! Just don't be afraid to start super simple.
Mark recommends the following next steps:
- Investigate Unity 3D
- Explore the Unity asset store
- Check out game courses for Unity on Udemy