In the early days when there were articles speculating on what Glacier was backed by, it was actually on crusty old S3 gear (and at the very beginning, it was just on S3 itself as a wrapper and a hand wavy price discount, eating the costs to get people to buy in to the idea!). Later on (2018 or so) they began moving to a home grown tape-based solution (at least for some tiers).
I'm not aware of AWS ever confirming tape for glacier. My own speculation is they likely use hdd for glacier - especially so for the smaller regions - and eat the cost.
Someone recently came across some planning documents filed in London for a small "datacenter" which wasn't attached to their usual London compute DCs and built to house tape libraries (this was explicitly called out as there was concern about power - tape libraries don't use much). So I would be fairly confident they wait until the glacier volumes grow enough on hdd before building out tape infra.
Do you have any sources for that? I'm really curious about Glacier's infrastructure and AWS has been notoriously tight-lipped about it. I haven't found anything better than informed speculation.
My speculation: writes are to /dev/null, and the fact that reads are expensive and that you need to inventory your data before reading means Amazon is recreating your data from network transfer logs.
I'd be curious whether simulating a shitty restoration experience was part of the emulation when they first ran Glacier on plain S3 to test the market.
