If you want to withdraw a certain amount of bitcoins from a cold wallet for a purchase or an exchange, you need to plug in your cold wallet to a device, usually a computer. Then you can transfer the necessary amount of bitcoin to a hot wallet, making it accessible to use.
Conversely, to send bitcoins from your hot wallet into cold storage, you will need to connect and unlock your hardware wallet, transfer the desired amount of cryptocurrency, and disconnect the device again for safekeeping.