First thing you need to understand that you have Bitcoin on the base layer (onchain), and you have Bitcoin on the 2nd layer (Lightning).

Onchain is slower (transactions are processed in blocks, generally 10 minutes apart) and more expensive. You send a transaction via your wallet, and confirm it with your key.

Lightning is fast and cheap. When using a Lightning wallet you can send to anyone else with a Lightning wallet by scanning a QR code and confirming with your pincode.

You can sell BTC on exchanges by sending it there or you can sell it Peer-2-peer via services like Peach, Robosats etc