Hi there,
I’m not sure if this is the best way to do it, but I recreated it with 4 shapes merged using Union selection - 2 rounded rectangles and 2 corner shapes.
The flow on the right shows I made the corner shape, then I simply rotated the corner to fit on the bottom-left side of the shape.
It seems my method was more complicated 😅 I realised that border radius is applied to both the corners and the edges where the objects meet. I learnt something new from you too, thanks!