Short answers, pulled from the story.
StageCraft is a visual effects technology developed by Industrial Light & Magic that uses large LED video walls to display photorealistic environments in real time. The technology was created to solve lighting and depth issues associated with traditional green screen filmmaking.
The first episode of The Mandalorian aired on the 15th of November 2019 and marked the beginning of the public use of StageCraft technology. The system was first tested during the development of The Lion King in 2019 before its full implementation in The Mandalorian.
ILM uses Epic Games Unreal Engine to handle real-time 3-D rendering of computer-generated imagery environments within StageCraft. A specialized rendering engine named Helios was also designed by ILM specifically for StageCraft hardware to manage complex lighting and shadow interactions.
Permanent StageCraft volumes are located at Manhattan Beach Studios in Los Angeles, Pinewood Studios in London, and Fox Studios Australia. A volume was announced to be built in Vancouver in November 2021 and was planned to be opened in early 2022.
StageCraft allows actors to see the world they are inhabiting in real time while filming, providing tangible environments that react to their movements. The lighting on the LED walls reflects onto the actors, creating a natural interaction between the subject and the background that is impossible to achieve with green screens.