To answer your question, right now it's
Concept Design.
Here is a quick rundown of the progression between phases.
(NOTE - this is only educated guessing on my part, so I may be a little bit off on some of the exact definitions, feel free to correct me if I'm wrong) I'll be using Galaxy's Edge as an example just to show progression between stages.
For reference, the initial BBTM concept art in 2022 with the little dark area was
Blue Sky - we might do this if y'all like the idea. Any art released is VERY fanciful. NO set details at all, things that obviously could not truly be built as shown, simply provided to give an idea of what could be.
View attachment 814433
This year it was
Concept Design - we ARE doing it, but we don't know a ton about the content yet. Still up in the air what actually goes into the land. Any art released takes much creative liberties, it's fanciful, will not accurately represent how it will truly look, but is there to show current ideas, plans.
View attachment 814431
Schematic Design is "ok we know what we want in the land, we know where it's gonna go, now we actually figure out how to make it practically work" - stuff about capacity, guestflow, shopping, return on investment, where BoH stuff goes, etc. It's also where they get the first models made, more permanent choices about ride scenes, building interiors/exteriors, creation of initial architectural renderings, etc. Any art released here is still subject to change but is far more likely to represent what will be built.
View attachment 814428
Design Development / Detailed Design is when they decide ok yeah we know how to make this work, we know what IPs are going where, now we make final design choices, make the layout for everything, final architectural schematics, 3d renders, model/final concept art released to the public, start finding contractors and transfer from WDI to final approval. The land really doesn't change much after this phase.
View attachment 814429
Then of course
Construction Documentation is exactly what it sounds like - you build it, maybe make last-minute choices or changes once you see it actually built, but this is construction and prepping for opening.
View attachment 814430
https://imagineeringtoolbox.wordpress.com/resources/the-imagineering-process-an-overview/ has more details.