IWTL how investment firms work, and how they can be used to launder money

Disclaimer: I'm probably going to end up on a list for this. It's for creative writing, I swear! 🤣

In my comic book series that I'm writing, I am developing a character who is a young finance bro, who uses his position and knowledge of stocks and investments to help several gangs launder their money.

The problem is that I have zero knowledge about investment, investment firms, security holdings, or stocks - let alone how they can be used to launder money. I do know the basics of money laundering, but specifically how that is achieved through investment firms is beyond me.

For the sake of realism, I want to learn more about this so I can write this character.

Youtube videos, (non-fiction) books, podcasts, articles, real-life cases are recommended.