These have been written about before on the internet. However, all the online guides I found were difficult to understand or contained incorrect information.
In this guide, rather than simply listing all unlockables, I am presenting information in the form of, unlock this by doing that.
Everything in the game is unlocked by achieving first place (gold trophy) on a specific cup in Grand Prix mode. If playing multiplayer Grand Prix, either player can get first place for it to count.
Cups and modes
- Special Cup: Get gold on 100cc Star Cup.
- All Cup Tour: Get gold on 150cc Special Cup.
- Mirror Mode: Get gold on 150cc All Cup Tour.
- Toad & Toadette: Get gold on 100cc Special Cup.
- Boo & Petey: Get gold on Mirror Mode Star Cup.
- Luigi's Mansion: Get gold on 150cc Mushroom Cup.
- Tilt-A-Kart: Get gold on Mirror Mode Flower Cup.
- Rattle Buggy: Get gold on 100cc Mushroom Cup.
- Para-Wing: Get gold on 50cc Star Cup.
- Toadette Kart: Get gold on Mirror Mode Mushroom Cup.
- Bullet Blaster: Get gold on 50cc Special Cup.
- Barrel Train: Get gold on 150cc Star Cup.
- Green Fire: Get gold on 50cc Mushroom Cup.
- Bloom Coach: Get gold on 50cc Flower Cup.
- Waluigi Racer: Get gold on 100cc Flower Cup.
- Turbo Birdo: Get gold on 150cc Flower Cup.
- Boo Pipes: Get gold on Mirror Mode Special Cup.
- Parade Kart: Get gold on Mirror Mode All Cup Tour.
Alternative title screen background
Normally, every single cup gives its own individual unlockable, except for 50cc and 100cc All Cup Tour, which have no associated unlockables.
However, to unlock the alternative title screen background, you must get gold on every cup on every speed, including 50cc and 100cc All Cup Tour.
I hate reading. Can you show me this as a picture?
I found a wonderful infographic created by user Zurioko.
Please see their post: https://www.reddit.com/r/mariokart/comments/qkznwx/i_had_a_lot_of_fun_with_the_infographic_of_mkw_so/ -- direct image link.