Sudoku+

Generally speaking for the whole app, its available in english and german, has a light and a dark color theme with the option for a custom one.
There also are explinations for most stages available, if one can't quite wrap their head around something.

When it comes to the default sudoku gameplay, one can choose between stages of difficulty (from 1-10) differing by needed solve methods and number of empty cells.
One can then solve the generated sudoku grid (guaranteed to have only one unique solution) useing a variety of helpful tools, such as the ability to note numbers, look at taken actions and reverse them, retry, check for wrongly set numbers and view methods of solving it if your stuck.

One has the ability to solve any grid, step by step with visual and text explinations (with explicit information for the given game),showing ways to complete it which one might have not known.

If the auto generated games on expert-difficulty are to easy for you, there are 40 extremely hard challenges available to test yourself against. Without the ability to check or solve it ;)

But you can also enter / make your own sudoku games, play, save and solve them. Enabling one to take a look at possable ways to solve games in newspapers or books for example. Which is made easier by photo recognition (-sloppy-).

Last but not least, one can play together with people in the same network. A contest for the fastest completion time, or solving a game together.