Yeah, it's all stacking. Lots and lots and lots of IGBT in series. Now though they have pretty advanced gate control, to build "Modular Multi-Level Converter (MMC)", switching various groups of modules on and off to approximate a sine wave. https://en.wikipedia.org/wiki/HVDC_converter#Modular_Multi-L...

Fun facts: the Trans-Bay Cable is credited by wikipedia at least as the first MMC implementation. Started ~2003, permitted 2007, completed 2010, the cable is 53 miles, and runs at 230kV, providing reliable power to SF; a response to a critical 1998 power outage when two local power plants shut down (since decomissioned for good) and the grid failed to carry the load. https://en.wikipedia.org/wiki/Trans_Bay_Cable

Coincidentaly, the original patent expired in 2003, so uh, interest may have started rising recently. None the less, thanks Rainer Marquardt and Anton Lesnicar for your MMC invention. https://ieeexplore.ieee.org/document/1304403