Comment by sweetheart

The "UI" is indeed represented in memory in tree-like structure for which positioning is calculated according to a flexbox-like layout algo. React then handles the diffing of this structure, and the terminal UI is updated according to only what has changed by manually overwriting sections of the buffer. The CLI library is called Ink and I forget the name of the flexbox layout algo implementation, but you can read about the internals if you look at the Ink repo.