Here is your modified ex with thickening the surface and the removal of internal material positioned after.
Otherwise compared to Mickael's answer, if there is a line appearing when you cut, it is because in the thicken function, the merge option is not selected. So there are 2 bodies. (Well, that's what I get on his piece.)
OPEP 27
Well done!! That's what I want to achieve as a result.
Only a stitch remains mysterious, why be forced to thicken and cut afterwards??
This means that if you make this modification on a hollow part with a lot of functions it will be a pain.
I tried on your modified part to make a surface offset from the surface you created and then to do the intersect function and it works (up to a minimum offset of 0.19mm) after that it doesn't merge anymore!
Thank you in any case for all those who helped me in this post!!
I positioned the piercing afterwards, because I didn't want to think too much either...
and it avoids having to redo a function.
Then you can also start from your body to extrude to the surface.