I have gone to the Affinity forum with example PDF above!
ABout the cylnder remark it's that i have seen!

>>Designer doesn't support Macros. Only Photo has them.

>>Your example file contains some curves, which reflect roughly edges which mark the surface of a 3D object.

I interpret your request that you want to get a closed curve for every visible surface area, build by using the edges and combining them to a closed curve (one edge is needed multiple times, mainly 1 or 2 times, for 2 edges who share the edge).

The grouping of layers (used for clipping) should be ignored for combining surface edges (you need edges from multiple layers groups for one surface)

Just „closing the curves“ (using the current „close curve“ function would not deliver any useful results.

the curves like the bottom from cylinder needs to be trimmed to cut of unused parts
This tasks needs some 3D functionality which is not at all available in designer.
The edges can be ambiguous (overlapping), and the layer sequence gives no actual „depth“ information.
Just to „manage expectations“ i personally rate this functionality very unlikely to come anytime soon. It is not a simple macro recording - which Walt already pointed out is not available at all in Designer - and record otherwise existing functionality.

otherwise, it would be a nice feature to get, of course.