Just from seeing (I can't read German), I do not know if he explains what he was doing, all I can go by is what I seen, so I can be a bit wrong about what he was melting on the fire brick, it looked like brazing alloy to me, being prepared to braze the rings with.
The first small butane torch looked like he was melting a small amount of metal, by the steps that followed I took this metal to be a brazing alloy, I assumed he hammered and cut it to solder or braze the ring with a small piece of this alloy, the small ring being heated enough by the small hand held butane torch to braze the ring together.
The second torch was larger The process looked similar as the first to me, the brazing alloy may have had a higher melting point, and needed a bigger torch a little larger brazing torch tip, the ring may have been a higher melting metal, and to melt the brazing alloy and heat the ring hot enough to braze the ring he may have needed the larger torch.
The last He was melting a large volume of metal, which needed a larger rose bud torch.
In brazing usually the brazing alloy will melt easier than the metal it is being brazed (soldered), although sometimes we can use the same metal, the flux helps to keep the metal and braze clean, removing surface oxides, fluxing the melt, helps to keep the surface of the molten metal from oxidizing so the brazing ally can stick to the ring, flow well into the joint, and bind the metals, most of the time in brazing just the braze is molten and binds to the surface soaking into the pores of the metal being repaired, but sometimes both the braze and the metal being repaired are molten and the two metals flow together and combine in the joint, this is normally not considered brazing but called welding, although it can be similar to brazing, like when the a brazing alloy is the same as the metal being welded.
It looked to me like he was using light weight low density fire bricks, for the fire bricks and melting dish made from the brick, these will not absorb heat well, or suck up all of his heat away from his torch or the metal he is melting. What you melt a metal on, or in, can be just as important, or sometimes more important than your torch, you could have a torch large enough and hot enough to melt a metal, but if that metal sits on a good heat sink you will not be able to melt the metal, the brick would suck up all of the heat.
A low melting metal can be melted with a smaller torch tip than a metal of a higher melting point.
A thin or small piece of metal can melt easier than a thick or larger piece of metal, the thick metal can act as a heat sink, and would need a larger torch tip of a hotter torch or different fuel/air or O2 to provide more heat.
Depending on the job, soldering, welding, cutting, brazing, melting, heating the metal to bend form or mold, we may need a different fuel, and or torch, we may also use different torch tips for different jobs.
Different jobs may need different amounts of heat, some we may need to get the metal just hot, like in soldering, some we may need to get the metal red hot, just below its melting pint, like brazing or bending, others we may need to get just a part of the metal to be molten but not the whole part, like in welding where both ends of the metal being welded together and the welding rod are all molten and we melt all three of these metals together at the joint, then there is where we need enough heat to get all of the metal molten as in melting a dish full of gold silver and copper into one molten mass.
We choose the tools for the job, torch, fuel, air or oxygen,torch type, torch tips, solder or braze or welding rods or metal, melting dish or heat sink, furnace....
To heat a metal we just have to get the molecules in the metal excited, to get it molten we have to make them rock and roll and get them excited about dancing a jig, we may need to play a different tune to make them molecules dance, and provide a good place for them to dance.
