>>>>How can i make spline, circles and rectangles flat?
From the link I referred in my previous post:
"- for circles - set center Z to zero,
- for polylines - set Elevation to zero,"
Note: Rectangles in fact are closed polylines. Circles and polylines are flat 2D objects, so you only need to put the center of the circle to XY plan and you need to set polyline elevation (Z level) to zero to make them "flat". That is correct for the case when polylines and circles were created in the coordinate system which XY plane was parallel to XY plane of World Coordinate system.

Splines are 3D objects by its nature, so you cannot use Properties palette to flatten them. So you can try to use another trick to flatten them:
https://thecadgeek.com/blog/2012/04/mac-flatten-command/