How to create a face on a specific shape?

I need help. The bottom of the arch should attach to the arch of the upper surface.
How to do it?

PushPull, Scale, Intersect Faces.

Basics of SketchUp at learn.sketchup.com

Use the edge tool to “connect the dots”, with diagonals added if necessary.

Thank you so much.

But there`s also this problem.

It will be easier if you first learn the basics of SketchUp. Do not work on such a small scale (mm).