Huge thanks to everyone that weighed in on my issue, I owe you all a beer!

It's some kind of plastic, but I can't tell the specific plastic type. Someone else suggested epoxy to me, I already added some silicone. I might add a layer of epoxy as a belt and suspenders approach.

From what I saw, I think you're right that it shouldn't be glued, but who knows what the plumbing guys had in mind. Talking to the other owners of the building, it seems like plumbing was a common weak point amongst the units.

I haven't been able to figure what that was. I must be an adhesive of some kind. It is completely hard, but there's a big drip so it must've been liquid at some point I imagine.

I looked at some installation videos for shower drain, and I understand what you mean with getting access to the tub under. Unfortunately, I think I'd have to tear down the whole shower to replace that piece.

I went with your recommendation of using shower-specific silicone. I can sleep more easily now. Thank you so much for the help!