Hello and welcome, @ROODvogel
Let’s start with the easy one… The arc Radii. Sometimes, you want to place a point at a certain distance from another 1 or 2 points. The best way to do this is to draw an arc from the points using the distance as the radius. If the distance is from 2 points, you’ll do this from the 2 points and where the 2 arcs intersect, that is where you’ll place the point - there is a tool for this (Intersect arcs).
If it’s only 1 arc, then the point will be at an angle to a point, in which case you will use the intersect arc and line tool and put in the angle that the line needs to intersect the arc at.
Curves: There has been quite a bit written on the forum about curve angles and control lengths and it’s really quite a difficult subject to explain. I’ve tried multiple times. The method that I always use for all curves, I’ve attempted to explain in this topic. I’d suggest having a look at the PDF file and attempting it yourself. If you have any questions, then I’ll try to explain. But I use variations of this method in all of my curves, everywhere in the pattern and my curves always turn out beautifully and resize smoothly.