Bitcoin isn't like regular money. This could make understanding where bitcoin is stored a little complex at first. Bitcoin is something that exists on a blockchain, which is just a digital ledger. The unique part about Bitcoin is that the blockchain that is uses has no central authority, which means anyone with Bitcoin can use it.
When we say 'has Bitcoin', it should be understood that all the Bitcoin there is exists on the blockchain. People can own private keys to Bitcoin, which allows them to send it when they want. These private keys are stored in a Bitcoin wallet, which is where Bitcoin could be thought of as being stored.
There are many kinds of Bitcoin wallets, and they allow people to hold Bitcoin as an asset. Because there is no centralized authority on the Bitcoin blockchain, if these private keys are lost or stolen, the Bitcoin is gone.
One great thing about Bitcoin is that the entire public digital ledger, or blockchain, can be inspected by anyone. This makes ensuring that it is a transparent system very easy, which is a big advantage over the existing financial system.
In a sense we can think about all the Bitcoin being stored on the blockchain, but it is important to understand that it can never leave its blockchain, as the only thing a person can possess is a private key.