Started investing in bitcoin using coin base pro. But as I look more into it most guides recommend storing your currency on an offline ledger or wallet. How do you protect yourself from a flash crash or 20% drop in value if its in a wallet?

Everywhere I see points to not leaving your crypto or cash on an exchange as they can get hacked, or your account can get hacked and your money taken. They recommend you use an offline ledger like the Ledger Nano X as an example, or a digital wallet. This is all fine but how do you protect your bit coin value in the case of a flash crash? Or do you need to choose?