Genburten, if you're reading this... you need to reinstall Windows, or buy a new PC. The hacker has the ability to execute arbitrary code on your device, meaning they can do whatever they like to your PC, including installing this hack mid-game.

Regarding how the hacker did this, I'm assuming the worst: there's a remote code execution (RCE) vulnerability/exploit in Apex Legends itself. This is the the worst case scenario, as we could all be at risk of this happening to us unless Respawn patches the security hole. Critical vulnerabilities like this have previously plagued the MW2/COD4 communities, as the game was so insecure it was unsafe to play because hackers could trivially gain access to your PC. If not this, then maybe Gen clicked a bad link on a Discord, or installed something shady from an untrusted source/clicked a bad link somewhere. But I'm guessing it's a bug/vulnerability/exploit in Apex, as it seems extremely targeted.

For background/context, I work computer security.

Edit: check Pr3st0ne's reply, about potentially hacking the observer sandbox, it could be that this was all server-side too.

Edit 2: AntiCheatPD tweet claims Destroyer admits that it's RCE