This picture is of a 1980 Schwinn Continental. I'm confident that the process hadn't changed at Schwinn in decades, so it is probably the same as a 1960's Continental and other bikes in their line.
Technically, you would be correct to berate me for posting a non-lugged seat cluster, but we like pictures and it does help to put the beauty of low production and custom frames into perspective.
It looks to me like Schwinn had some way to cut the top tube into a "fishmouth" then flare it in order to get enough contact area to braze it to the seat tube. If you look carefully you can see the gold color between the seat tube and the top tube. If I would have known, I would have done a better job of removing the paint at this joint. This is not the prettiest way to attach a top tube to a seat tube, but Schwinn was making bikes in high volume with a lifetime guarantee.
The seat tubes have a ball end. This is almost a Schwinn signature look for their seat tubes whether they end at the seat cluster or the down tube as on cantilever frames. They all had this same end on Chicago made frames. I suspect they took the cut end of the tube and roll-formed it into a ball end.