A utxo is an unspent transaction output. More specifically it’s a locking script and on amount. Bitcoin wallets tally up all the utxos you have the private key(s) to unlock(spend) and this is your balance. If you do not have one or a combination of many utxo’s that add up to more than the amount you want to spend than you cannot create and sign a valid bitcoin transaction.
Because Bitcoins are not "created/made" but instead it is "distributed" through Coinbase. The total epoch with block reward as incentive are fixed to the first 34th epoch (.2140-200
Speaking of 21 millions, there could not be "21 millions Bitcoins" either in circulation or in existence, why ?
The supply cap is less than that
OP_Return or some other scenarios can make UTXOs unspendable
Some Block rewards aren't fully-claimed (intentionally?)
A Bitcoin is divisible up to eight decimal places. There are actually 2,099,999,997,690,000 (just over 2 quadrillion) of maximum possible atomic units in the bitcoin system.
The value of "1 BTC" represents 100,000,000 of these. In other words, each bitcoin is divisible by up to 108.
As the value of the unit of 1 BTC grew too large to be useful for everyday transactions, people began to negotiate with smaller units such as milli-bitcoins (MBTC) or micro-bitcoins (μBTC).
It's tuesday, your friend tells you they would like to hang out next friday. Does that mean they want to hang out in 3 days or in 10 days ? is next friday the literal next friday or the friday in the next week? :)