Logic Masters Deutschland e.V.

This is my 30th entropy sudoku in the portal. To celebrate that, this puzzle is intended to show in a more accessible way what I like so much about this constraint myself. It's not scanning for 2x2s where one group isn't represented, but anything that requires thinking beyond 2x2. The little tricks and techniques that make up the elegant and unique logic of this variant. This puzzle is designed to help you learn a few of them. They are the tools to be able to solve the difficult entropy sudokus.
There is also a hidden hint under the puzzle on how to think in order to discover the tricks and techniques mentioned.

Rules:

Sudoku: Fill every row, column and marked 3x3 box with the digits 1-9.

Killer: In cages marked with a black dashed line digits must not repeat and have to add up to the total given in the top left corner of the cage.

Entropy: The low digits (1, 2, 3), middle digits (4, 5, 6) and high digits (7, 8, 9) are defined as three different groups. In the finished grid each 2x2 box has to contain at least one digit of each group.

Have fun solving!

Sudoku on:
CTC-App
f-puzzles

Hint (Highlight the white text below to read):
The entropy rule suggests that it is essentially about scanning 2x2 squares in the grid, which of course is a big factor in these puzzles. But the fact that due to Sudoku rules only 3 digits of each group can be placed in a row, column or 3x3 box creates further limitations where larger areas have to be considered overall. The best example are the 3x3 Sudoku boxes. Many arrangements of two digits of a group in a 3x3 box allow to place the third digit of the same group either exactly or at least limit its position to a domino of cells.