Thanks for that, that's a very recent change. Still just tacks. But I suspect that it's all going to be welded together once the thing is fully assembled; I can't imagine these tacks holding during launch. Given that they're going so far as to cosmetically clad the bottom part, then they probably will polish the seams (it's a lot less work intensive to polish seams out than whole sheets).
That said, it's still not going to look like an optical mirror like in the rendering.
A curious process for the bottom portion - assembling the loadbearing structure and then cladding it. I guess they can't get the sort of optical qualities they want (which are not just about aesthetics, but also heat rejection) with their structural steel alone?
ED: Something just occurred to me. And it really should have been obvious, but... the reason for the bottom portion being built so much more sturdy than the top is... this is a Starship hopper they're making, not Super Heavy. Starship's passenger / cargo volume is
huge. That
whole** "flimsy" part on the top is the passenger / payload enclosure. Tanks are only on the bottom, aka the "sturdy" part. The final Starship will have bigger tanks, but both the test and final version are having a full-sized passenger / cargo area. Geez that's a lot of cargo space....
** - Even if you assume that the upper bulkhead will arch upward from the bottom structure and eat up part of the space.
This should have been obvious, but it just now occurred to me as to why tanks wouldn't fill up a lot more of the structure: the cargo section is sized the same as in the final version, and it's monstrous.