A computer which runs the bitcoin-core software.
Every bitcoin node contains it's own copy of the entire bitcoin blockchain. In most cases any two bitcoin nodes might differ in the last 6 blocks of the bitcoin blockchain.
A collection of unverified transactions. Every bitcoin node has it's own version of mempool.
1 BTC = 100 million satoshis = 1^8 satoshis.
A user who successfully mines the next block is rewared with certain amount of satoshis. Initially the reward was 50 BTC.
A transaction which assigns a fixed predetermined amount of satoshis to the address of the miner is called the coinbase transaction. Every block has exactly one coinbase transaction.
Once there was Satoshi. He ran the bitcoin software first and mined the first block of the blockchain. Initially there was no bitcoins, so the first block had only one transaction, which is indeed the coinbase transaction. Which is the reward transaction, basically satoshi assigned himself 50 BTC to his public address. Which means once the first block is confirmed(accepted by the bitcoin network), there was exactly 50 bitcoins in supply.