Sudoku

My wife got a new phone, and the sudoku app she had on her old one is no longer available. Do any of you have recommendations for an Android app that doesn't do ads during the game and can be played offline?

My understanding is a valid sudoku should have 1 unique solution. But in this one I just finished, the circled numbers seem to be interchangeable. Am I missing something?

I typically get stuck around this point. I begin with penciling in which numbers pinky have two options per 3x3. If that dries up, I will try looking for numbers that can only go in one spot per row/column and squares that only allow 1 number.

Are there powerful techniques I'm skipping?

Not sure where to go from here. I'm still relatively new to suduko so are there any techniques I could use here?

Any hints appreciated

I am stuck on this puzzle. Any suggestions for how to move forward? Thank you.