If there were no bitcoins at first, how did a miner approve the transaction in the genesis block? I mean if there are no mined bitcoins (balances), how can a transaction take place? Also, I know that first reward of the PoW was 50 BTC which was transferred to Satoshi, and this 50 BTC is not spendable.
The coinbase transaction in each block creates new coins and as a result does not require any pre-existing balances. You can accept new coins in an address even if you have no balance. The fact that it was the genesis block is irrelevant as you can still mine any block without any transactions but the coinbase transaction.