What happens to crypto assets held in your Coinbase account?

I'm curious to know, what exactly happens to the crypto assets that I hold in my Coinbase account? Are they stored securely? How do I ensure the safety and security of my digital assets? What measures does Coinbase take to protect against potential threats or hacks? Also, how do I access my crypto assets when I need them, and are there any fees associated with withdrawing or transferring them?