Post a diagram of what you are envisioning. But, a wall won't be sufficient. You'll need LOTS of custom steel.

What material will be used for the countertop?

Oops! I forgot to mention the countertop material: probably 3cm granite. the floating countertop/seating area is actually L-shaped. Pls let me know if I am missing any critical dimensions.

1/4" steel won't be enough for that. You need table legs. Or a different plan. Or someone to weld up some custom tube steel that goes from under the granite down through the floor, well bolted to the foundation.

Sophie, so brackets from somewhere like countertopbracket.com would not be enough? Is this because of the depth of the countertop?

From the photos/description on their website, it looks like there are some brackets specifically for this purpose. I am curious why this would not work.

3' of cantilever corner won't work with ''brackets''. What is your foundation type? How do you plan to transfer that 900 lb weight safely down to that foundation?

How do you plan to secure the stud wall to the foundation? How do you plan to enssure that a wood stud wall doesn't flex under the weight of someone standing on cantilevered stone supported by metal brackets screwed into flexible wood? Stone won't stand for any deflection at all. It WILL crack.

Make that a structural steel wall, and bolt it and epoxy it to the concrete, and now we're talking. And use something other than flat 1/4" steel under the stone. That still will flex, given the weight.

Are ultra-compact surfaces (eg dekton) lighter than granite? If yes, would using such a material solve some of the problems?

Some 1" square tube welded into a frame and installed across the cabinets and wall should be fine.