Trivially, the spell text does not say that a simulacrum knows what its "original" knows. Spells only do what they say they do, so the simulacrum doesn't know what the original knows.
For instance, the Clone spell explicitly states that the soul-infused clone keeps its memories (emphasis added):
The clone is physically identical to the original and has the same personality, memories, and abilities, but none of the original’s equipment.
Simulacrum does not have this language, so it doesn't retain its memories. It's also worth noting that this text delineates between three key aspects of a character: personality, memories, and abilities. While a simulacrum might know just enough to use its abilities, as detailed by its game statistics, it would not have memories like the location of the cult's secret HQ.
It's not specifically stated in the spell, but the wording of the spell suggests that the simulacrum is a different creature that has the same game statistics and appearance as the original.
Below, I've bolded the places in the spell that show how the duplicate is different from the original:
You shape an illusory duplicate of one beast or humanoid that is within range for the entire Casting Time of the spell. The duplicate is a creature, partially real and formed from ice or snow, and it can take actions and otherwise be affected as a normal creature. It appears to be the same as the original, but it has half the creature's hit point maximum and is formed without any Equipment.
Thus, the simulacrum has the appearance of the original creature, but nothing else.
Moreover, just because two creatures have the same game statistics doesn't mean they have the same knowledge--you can have two wizards that have the same stats but different knowledge, for example.
As for spells, as you point out, the text refers to expended spell slots, which seems to assume that the simulacrum does keep the spellcasting class feature of its original.