Collect every Player Cartridge.
Player 1 - Leave the Core!
Complete the game and achieve the true/bad ending.
Player 2 - Beyond the World
After the sequence where you chase the mini people, you'll take a ladder to reach an endless hallway. Turn left and keep moving forward until you see a white box on the ground. The Player 2 Cartridge will be on top of the box.
Player 3 - The Basement
When you reach the Basement, walk down the stairs and enter the first room you see in front of you. Turn right to find the Player 3 Cartridge on a shelf beside two jars.
Player 4 - The Loop
In loop 14, check the white table on your left right as you enter the room.
Player 5 - Mini Mita
This chapter starts with you helping out Mini Mita in a miniature house/forge. After crafting the Giant Key, you'll be able to use it to unlock the door on the left to reach the 1.15 Game World. Head towards the large bookcase in one corner of the living room and pick up the Player 5 Cartridge from the top shelf.
Player 6 - Reboot
Play through the chapter normally until you reach the point where you go back down to the Basement to enter the Player ID. Instead of using the Computer, go up to the Portal Console. The Player 6 Cartridge will be waiting for you on the Scanner.
Player 7 - Reading Books, Destroying Glitches
Destroy the second glitch in the bathroom. Leave and then head back inside the bathroom and pick up the Player 7 Cartridge sitting on one of the shelves near the door.
Player 8 - Old World
Once you reach The Core in the chapter Old World, turn left immediately after entering the restricted barrier. You'll find the Player 8 Cartridge on the chair in a blue bucket. Look carefully, it can be pretty hard to spot in the dark.
Player 9 - Old World
This one is collected earlier in the chapter. After Crazy Mita leaves, head into Creepy Mita's Bedroom. Walk over to the Computer Table, and you'll find the Player 9 Cartridge on a Checkered Box near the shelves.
Player 10 - Be Candid
After starting this chapter, head straight to the bedroom and walk over to the nightstand by the bed to find the Player 10 Cartridge.
(Solutions edited via
https://gamerant.com/miside-how-to-find-all-player-cartridge...)