When the computer wants a bit to be a 1, it pops it down. When it wants it to be a 0, it pops it up.
If it were like a punch card, it couldn’t be rewritten as writing to it would permanently damage the disc. A CD-RW is basically a microscopic punch card though, because the laser actually burns away material to write the data to the CD.
Current ones also store multiple charge levels per cell, so they're no longer one bit each. They have multiple levels of "punch" for what used to just be one bit.
What the Romans had wasn't comparable with an industrial steam engine. The working principle of steam pushing against a cylinder was similar, but they lacked the tools and metallurgy to build a steam cauldron that could be pressurized, so their steam engine could only do parlor tricks like opening a temple door once, and not perform real continuous work.
Just about all of the products and technology we see are the results of generations of innovations and improvements.
Look at the automobile, for example. It's really shaped my view of the significance of new industries; we could be stuck with them for the rest of human history.