Understanding the TypeScript Variance Pattern in Effect Source Code

skimming through effect source code i noticed the use of the following pattern,

  [TypeId]: {
    _A: (_: never) => _
  },

  [TypeId]: {
    _A: (_: never) => _
  },

it looks related to the variance of the underlying stucture, but can some explain what's going on here?

Understanding the TypeScript Variance Pattern in Effect Source Code - Effect Community