Uneven walls, not solid I'm using Cura 4.5.0 and an Ultimaker S5

The walls in my model are coming out not solid. I don't have a clue why.





When I open the STL in Sketchup it has all these weird lines. I don't know how they got there and I don't know how to get rid of them.
Triangles?!
First of all, the triangles on the STL are not a problem - they are just how STL is defined: a mesh of triangles. you can't save anything but triangles in STL, so let's not bother with that item but the actual elephant in the room: the print.
Underextrusion
Step 1: proper settings
It shows signs of underextrusion. And I can exactly tell you where part of it comes from: You have set the line width to below the nozzle width. However, the line width should be best 10% larger than the nozzle. All of these lines should read between 0.4 to (as I have set it) 0.45 mm:

With a 10% wider line planned into, it is not necessary to have an extra initial layer line width of more than 100 %, but it can help in adhesion.
Step 2: Other issues?!
There might be other issues at work, though they will show up after setting the width much better. The following two strike me as most likely if the problem persists with the now considerable increased flow:

The retraction and/or retraction speed might be set too high.
mechanical issues of the extruder system, for example, worn gears or uneven pressure against the gear due to damaged parts.