Your private keys are stored in your computer like people here already replied .

However,  your btc aren't really in your computer. Your bitcoins are stored in every full node, I.e., every computer running a bitcoin core or some other implementation.

This is why it is called decentralized.  Your bitcoins cannot be deleted if the folder is deleted

What makes your bitcoin spendable only by you are the private keys. Although your bitcoin are everywhere, they can be spent by anyone  and anywhere just by using the private keys .