According to my interpretation of the drawing, the lines are what are called structural lines, in the terminology of an interpolation.
- Structural lines are vertical jumps in the terrain.
They are not necessarily constant level lines.
In manual interpolation, the structural lines should be considered in pairs, a line that represents the upper levels of the structure and a line that represents the lower levels of the structure.
The contour lines adhere to the lower structural line as they reach the levels it represents. They are grouped on top of each other while the structure is vertical, and they begin to separate from the upper line as they reach the levels that the upper line represents.
I think you should start digitizing these lines as simple closed LineString features. It is not necessary that you immediately worry about its level, since it seems more appropriate to define it later.
And digitize the known level points as point features, probably with their Z value included in the geometry, or in a separate field of the attribute table.
I am particularly interested in understanding in the best possible way how the restriction of structural lines is applied in the interpolation algorithms of QGIS.
If you could digitize some representative lines and points of known level, for at least some sectors, and share those layers in a geopackage file, avoiding us having to interpret the meaning of the different numbers and nomenclatures that are currently present in the drawing, I would be delighted to strive to investigate the application of structural lines to QGIS interpolation algorithms, try to build a DEM that represents the topographic condition of that terrain in the best way I could, and share the procedure used.