It starts, as you suggest, as a much larger tube, and then is drawn to the correct diameter.

https://static.makeuseof.com/wp-cont...er-640x240.jpg Is a picture of single-core solder. 5-core takes a little bit more care, and will not be possible with the more brittle formulas.

The key is keeping the tin/lead admixture annealed during the entire drawing process, yet not so warm as to melt. Noble metals do much better at this than others, and lead is very nearly a metaloid in any case - so this is not a casual exercise. The multiple cores serve to spread the rosin more evenly, so it is not just hype.

Peter Wieck
Melrose Park, PA