I understand how to send BTC to cold storage or offline wallets, but I can't seem to find information on how to send BTC from cold storage to, let's say, a wallet on my computer, or a website, or a friend's online or offline wallet.
I have successfully used Electrum wallet with cold storage. I found this one of the most novice friendly options, though getting money out from the cold storage is still quite a complex task.
Specifically, here is a tutorial how to create a two wallet: online and offline. Then you can transfer bitcoins from online wallet to offline wallet.
When you transfer bitcoins back to online wallet, you create a raw transaction on online wallet, then go to the offline wallet to sign it, bring the transaction file back to online wallet on USB stick and broadcast it to bitcoin network.
You should import the private key using the appropriate feature in whichever software you use. Then you can send the coins wherever you want.