Mask ROM = using UV masks containing the prepared ROM data to etch the photoresist for the metal interconnects of the chip permanently at integrated circuit manufacturing time to represent the data pattern desired. There is nothing to wear out or degrade. They literally last forever.
Applying a specific address will always route through the ROM matrix the exact same way and produce the same results on the data lines every time. It's literally as hard wired as an ASIC can get.
Imagine you have a prefabbed but unfinished IC chip designed, all it does is take an address and look up a data cell from an array. You have it wired so all the rows and columns of every cell are identical are permanently "1". These are pre manufactured and set aside.
Now your customer Nintendo comes along with the data they want in the form of a lithographic mask containing holes on it that represent binary data. You apply a final layer of photoresist to your wafer, expose it to UV through that mask, then etch away the non "fixed" photo resist and exposed metal layer. Any place that had a hole stays a "1", any place without a hole becomes a "0" because the photomask wasn't exposed to UV and "fixed". It allows the interconnect at that location in the grid to be exposed and broken by the acid etching. There are millions of the "holes" and "non holes" on the masks. The entire 4, 8, 16, 32, 4096, whatever megabit ROM is all completed at once.
Now your IC is finished and can be sent off to be broken apart from the wafers and sent to final testing and assembly and packaging as a familiar black epoxy chip with legs.
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