8/k7/8/8/8/7P/2r5/1R5K w – – 0 1
Consiser 1.Rb3 side by side with 1.Rb4. Then you can explain why 1.Rb4 works. I don't want go through the entire line you give, Alena, but I think black can force white into a Q vs R ending with best play.
I had to work out a few variation to understand why they don't work before I could find the first move. I considered Rg1, h4 and Rb5. After that I could understand the main idea of this puzzle.
It's a winning position for white.
The idea is actually quite simple. But it is difficult for amateurs to understand the move sequence. This is a very good exercise to learn R & P endgame.