Inventory can be found in the My Team tab. In this section, you can customize your characters as you like. You can change their equipment (shoes, jersey) as well as their dunk animations, victory moves, entrance moves and the emojis available during matches.
To customize your characters, go to My Team -> Inventory -> choose the character you want to customize and simply move between tabs and choose what you like. Remember that stars don't share their equipment, and each piece of equipment can be used by only one character. You can make your characters wear the same equipment for a better team sense or make them all look different to show off individually.
You can obtain new inventory items by progressing in career mode.