You send: Wrapped Bitcoin (Polygon)
Wrapped Bitcoin (WBTC) is an ERC20 token backed one to one by bitcoin held in custody, made to bring BTC into Ethereum applications. Its value tracks BTC; its address format, network and fees are Ethereum's. More about Wrapped Bitcoin.
On this pair it moves over Polygon — an address there starts with 0x — 42 characters.
WBTC also exists on other networks — different addresses, not interchangeable: Arbitrum, ERC20, Optimism, Solana.