I have completed this puzzle with the help of hints from the app. But I am not able to figure out the logic. I normally proceed by placing sure (unambiguous) digits first. Then repeat. But here I am not able to get any sure digit.
Please help with the explanation.
This puzzle comes from the "Sudoku.com - Free Game" app for Android available on Google Play here:
This puzzle is one of the daily challenges for the date 6th May 2019. The first picture posted is a partially solved puzzle.
I know the partial solution is correct as I have the app configured to automatically report incorrect entry as soon as I fill it.
The unsolved puzzle is given below: