I thought magnetic tape was kind of the standard for this type of storage, would be curious about the details of this system and how the drives work

My understanding is that if you delete your Facebook they will, in fact, completely delete it. That's post Cambridge Analytica of course.

One petabyte may have looked impressive in 2014 but is it still (honestly asking, it's not rhetorical)? A friend of mine has got 200 TB in his homelab. That's one fifth of the way there.

One petabyte is definitely within reach of an individual nowadays. Granted, it's only HDDs and not Blu-ray discs, but it's still a manageable amount.

Now as to why an individual would fill that is another topic: scientific data or options trading data or the various blockchain transactions data (including what's happening on the "L2" chains/sidechains) would quickly fill that, for example. But that is not my point.

Is one petabyte a lot? What would be a lot today?

For although 10 000 Blu-ray discs is something, 100 000 or one million discs to manage is something else altogether.