DecodeChess is using Stockfish 12 NNUE. and is running on our servers. The depth is usually around 24, but it can change dramatically, depending on the complexity of the position. For example, simple positions can reach a depth of 48.
When clicking on the Decode buttons (Decode Game or Decode Deep), the engine is run on our servers.
The more basic analysis interfaces available in DecodeChess, such as the Game Map and the basic analysis (Fish icon below the board), are running on your browser.