Earl’s Court Square – Fascinating Building, how did they achieve the Glasbox on the front?

Yesterday I saw this building in Grand Design: House of the Year: https://youtu.be/nrCWBxyokIA?si=5PwqqJzW754AtqET&t=2893 and I was completely fascinated by the construction of the Glasfront with Windows and Roof meeting at the edge with nearly no visual profile.

At least for me there is also no perceivable way how rainwater leaves the Glas as it seems to be compeletely flat. So far I found 5% as the minimum for Conservatory Glasroofs for example.

Can anyone explain a bit more how they achieved it or point me to some ressources?

I expect that its prohibitivly expensive to construct but I would really like to learn a bit more.