An addendum to the previous replies:
Bitcoins do NOT actually exist.
There are no such things as Bitcoins on the protocol level; what exist on the blockchain are UTXOs (unspent transaction outputs)
You've heard that the blockchain is a "ledger"
A ledger is a collection of accounting entries consisting of credits and debits.
That's how the blockchain works.
On a ledger, money doesn't exist; what exists are credit or debit (transaction) entries.
Bitcoins "exist" only as transaction histories on the blockchain, so you could say that bitcoin is a form of "triple entry accounting"
This transaction history is stored on the hard disc drives of everyone that runs a full node.

TL;DR: Bitcoins aren't stored anywhere because they don't exist.