Thank you Michael e Grazie a Mauro.

I made it! Indeed it was a matter of joining the pieces at each major step and at the end "union"izing them to get a solid.
Price for 3DP ceramics is still too high, but making it smaller won't make sense as it gets too deformed.
Thank you for the advices.
I thought the splines were in my attachment as hidden curves.
I'll be more attentive next time

Enjoy your Sunday
Monica

PS: apology for lack of accuracy in my wording ........but 3 months ago I didn't know the meaning of extrude and fillet.