quantum is from the latin root for amount - quant- and has the modifier -um meaning a thing (it is much like the greek -on which also is where the word one and in come from).

so it means, essentially, the atom of something. the smallest indivisible thing that a phenomenon involves. the word "atom" got lost in all this because we figured out that atoms were divisible into smaller parts. but a quantum, as in quantum physics quantum, is the actual "atom" of physical reality, and it turned out that this "atom" thing also has a connection to information, which is intangible (so matter is based on something that is intangible, take that materialists).

the block is the quantum of bitcoin, according to the definitions i just presented. you can talk about how it is divisible into many parts, and say "but that's not atomic" but it actually is atomic. how can i say that?

can you divide the process of bitcoin into smaller units than a block? in terms of time. the answer is no. the block is the clock, the clock is the block. the difficulty adjustment is purely based on the timestamps on the blocks, not on when the node received them, because this information is also atomic, whereas every node sees the block at a different time for the first time, that is metadata, not intrinsic.

you can say that the block represents a snapshot, a quantum of bitcoin, but what goes into producing it is not, it is extremely disorderly and concurrent.

the atomicity of blocks is because of the physical limitations of the internet. there has been attempts to make the transactions the atom of the ledger but this doesn't work with the increase of latency of the system. it's a consequence of decentralization being a key design target.

even though you can point at a mempool and say "this is the process of building a block" what the block actually is, is the mempool and block header of an as yet unknown miner who will mint the next block.

also, the UTXOs are extrinsics, compared to the blocks, when you talk about bitcoin as a system. they are what users put into the blocks, and create part of the incentive to mine the blocks because of the transaction fees, which the miner will reject if they are below their target threshold.

the UTXOs are atomic, in the sense that at any given block height, everyone agrees that a certain pubkey hash controls it, but their quantum is the spend, which is the next UTXO that will be built on top of it, so it is ultimately subordinate to the block in terms of ontology. the block is the quantum of bitcoin, the transactions are the state in that quantum.