That is correct for the bitcoin network difficulty.
But each pool has its own smaller difficulty for each miner because it is neither possible nor necessary for the pool to check every single hash.
For solo mining the Apollo runs a "mini pool" in the background.
... each pool has its own smaller difficulty... - This one has just blown my mind. What does that even mean?
To my understanding the pool would check a single hash ONLY if it is submitted as a valid block. There is simply no other reasons.
I know that Apollo runs its own pool, but I don't understand how could that pool have its own
smaller difficulty that is different from the bitcoin network difficulty? It makes no sense.
The network difficulty increases as more/faster miners find valid hashes.
If by "the network difficulty" you mean the bitcoin network difficulty, then of course I understand this as it is a core principle of the bitcoin network, but I cannot correlate it with the "Best", "Accepted" and "Rejected" Shares on the Apollo's Dashboard.