A decentralized app / Dapp is NOT distributed, it IS NOT running on multiple computers, and it may or may not have the peer to peer option. It IS a computer program which runs on a single computer - be it a notebook, tablet, phone, raspberry pi system, etc. It IS NOT dependent on the consensus protocol of the blockchain, or proof-of-work. A request send from a Dapp, if valid, IS always processed by the node and may be incorporated in the next block. The actual way of calling is by HTTP GET/POST requests to 'localhost' - for ex.