I'm 100 percent certain that this is a spacecraft or launcher fuel tank. I've worked on reentry analyses of spacecraft fuel tanks before, and I can tell you that these very commonly survive re-entry for several reasons.

Firstly, they are by far the largest components housed inside the vehicle's outer structure. The structure usually takes the brunt of the aerodynamic and thermal forces, protecting the tank from the largest destruction.

Secondly, the tank itself has to handle the huge fuel pressures involved (easily going towards many hundreds of atmosphere pressures!). This obviously means that incredibly tough materials have to be used.

Thirdly and finally, as can be seen in the pictures, their shape is symmetric, making the shape aerodynamically very unstable. This means the fuel tank has the tendency to tumble in the airstream. The tumbling continuously causes different parts of the tank to be exposed to the heat and other parts to cool down.

Fuel tanks are the major risk during reentry to people and other stuff on the ground, perhaps together with the massive engine blocks. To minimise the chances of hitting someone or something, re-entries are nearly always aimed at the Pacific Ocean, hence them being more likely to wash ashore in Australia.

better article.

What's the fibre pattern on the dome part I wonder? Looks like some kind of carbon fibre maybe around the top in the video.

I wonder why it was buoyant. Like if it's just the sheared off end of a cylinder you'd think it would just sink?

IDK enough about anything to say "I reckon it's x", but if it's not rocket parts then my guess would be some kind of reusable buoyancy tank used for things like floating foundations into place, shipbreaking yards, that sort of stuff.

looks like some solid fuel tank