Inkscape (=freeware) has several tools for this
- stroke to path (=convert a curve to fillable area which can have a new outline stroke)
- offset path (=a copy with some selectable distance)
- path function "pattern along path" (arbitary shape follows a separately drawn curve)
- bending a shape from clipboard along a curve as you draw it
The first option is the simplest. But the curve loses its editability as a curve, so you need a spare copy which is not converted.
The last option is demonstrated here:
Three lines are drawn and aligned
The middle line has got a blue, much wider stroke and the pattern is grouped & copied to the clipboard
Three different curves has been drawn with freeform curve tool and the pen. All have option "Bend from clipboard" ON.