Thought I will need only wallet passphrase for sending bitcoin. Will I need private keys for sending bitcoin as well?
You either need you wallet file + passphrase or only private keys to spend your funds. The wallet file contains your private keys and it is encrypted with your passphrase.
Also when I send coins, which wallet will I use? One of my depo addresses? or what?
One or a few of your addresses will participate in the transaction depending on the number of coins you want to spend. Let's say you want to send 0.15 BTC but you received 0.1 BTC to two different addresses. Both of these addresses will participate in the transaction and the remaining 0.05 BTC minus the transaction fee will be returned to your change address.
Got it. Thank you!
Last question then. Bitcoin core already comes with wallet.dat in its wallets file. Do I need to do "File" and "Backup Wallet" again?