Is there any way to select all the lines in the whole circle？

Another question: when I want to draw a rectangle on the midpoint of a circle, how can I show the midpoint?

Is there any way to select all the lines in the whole circle？

Another question: when I want to draw a rectangle on the midpoint of a circle, how can I show the midpoint?

Is there any way to select all the lines in the whole circle？

This would need edge loop selection feature which is not available yet unfortunately. Might be a possibility in future.

When I want to draw a rectangle on the midpoint of a circle, how can I show the midpoint?

Is the circle on the same plane as your rectangle? If you select your circle the centre point will appear, but you need to draw your rectangle on the same plane.