A leyline consists of three parts. First a source. Next, a stream. Finally, a taper. A source is a single point from which the leyline originates. The source cannot be moved, it is locked rigidly in place.
The stream may be moved laterally, but it cannot turn sharply, so in order to move a leyline's path by a significant distance, one must travel along its length and push the leyline gradually in the desired direction. The energy of all leylines are drawn from the strength of the love for the land of its inhabitants.
The taper is the furthest area where a dragon, who is aware of the location, and regularly visits it, lives.
When a dragon or a being with a sufficiently draconic soul forms a significant spiritual connection with a place or a geographical feature, two things happen. The weight of the emotional connection settles into the ground beneath the place, and particles known as “faespores” settle into the ground, pressed there by the weight and significance of the emotional connection. These faespores accumulate over time, as the dragon makes repeated visits to the place, as the dragon talks to the place, as the dragon watches over the place. Eventually the faespore density reaches a critical point, at which point the spores germinate, and the rhizomes (which are in a higher physical dimension but whose weight and 'signal' impacts the third dimension) spread and interweave, forming the physical leyline. The emotions accumulated over that ground fill the leyline like electricity in a wire, and a nature spirit, the field projection of that emotional current, comes into being somewhere in the area around the place. The range the nature spirit may freely roam varies depending upon the strength of the emotional connection, whether there is more than one soul with a strong bond to that place, and whether other souls (like those of sapes or kobolds) also have a strong connection to the place.
A nature spirit may leave the place and roam freely if an oath of return is performed. The dragon's own blood joins the leyline, and a pinch of the soil is swallowed by the dragon. If the place is somewhere there is no soil, substituting soil for a fragment of the ground, or a handful of the water, will also perform the same function. Failing to return to the place of origin before the death of the dragon, or the death of the nature spirit, will banish the souls of both to the higher dimension where they will be gradually and eternally pulled apart where the rhizomes lie (in place of reincarnation or an afterlife of any kind).
Additionally, in the event of a broken oath, the leyline will cease to exist, reducing the soil to ash and destroying any natural beauty that may have once inspired a connection.
As for how magic is drawn from a leyline, this is done through meditation on the ground where a leyline covers, within the area the nature spirit of that place may roam. As for the strength of the leyline, this may increase gradually over time (and usually does) if people settle in or near the area and show reverence or respect to the place where the leyline is sourced.