What kind of data is stored in the RAM when it is fresh out of manufacture? Apart from certain basic instructions (if yes, what are they), are there numbers stored as well or are most of the memory cells just empty?
RAM is cleared after each power cycle. The power-up value of cells depends on the technology.
Static RAM content is usually random (there is no preference between 0 and 1, but the same chip may present similar patterns after each power-up).
Dynamic RAM has the value corresponding to empty capacitors in the RAM array, which usually corresponds to ones, but there is no guarantee. Dynamic RAM can retain data for several seconds when unpowered (very dependant on temperature) : One cannot assume a known state at power-up and recent DRAM (DDR3, DDR4...) need complex initialization sequences where the RAM controller stores and checks some values like FFFF/0000/AAAA/5555, ...)
Memory cells are just electronic components whose voltage is used to store information: you can think of it as high voltage meaning a binary 1 and low voltage meaning a binary 0. Everything that is stored is either binary 1s or binary 0s.
As such, there isn't really any such thing as "empty" and, as far as RAM goes, there isn't really any voltage if the memory isn't connected to some kind of power source.