Greetings and welcome to the community forum!
There is a "freeform line" tool; I think this may be the one you've already found. You can avoid having to re-start it by starting with a double-click. This makes the same tool available for multiple uses, until you hit escape or choose a different tool. You may need to undock the line tools palette to get a free-floating window, in order to allow double-clicking.
But that's still pretty limited for drawing. It might help if we knew what sort of work you need to do and what the goal is.
Is this what is meant by vector drawing and is only for drawing ... mathematically calculated lines?
That is exactly what is meant by "vector drawing". The graphic objects are all stored as numerical data (points in an xy plane) and connected by lines and curves calculated from the points. That doesn't mean it can't be used for more artistic, free-style projects, but there are limitations.
If you want something closer to painting, you probably need some kind of bitmap graphics tool, which OO Draw does not provide.