(1) overall selection.

(2) Use the direct selection tool to turn the corner polyline into a curve, which can be achieved in one step without disassembly. The specific way is: after entering the direct selection state, there will be such a situation-a small circle appears in the corner:

Then use the mouse to gently drag on any small circle, and it will be like this (the lighter the drag, the smaller the radian of the angle, the greater the radian):