Fillet brazing is when the frame's tubes are very precisely mitered to interlock perfectly, and then basically "glued" together with molten brass or silver. This works via capillary action, and it's basically the process used for lugged frames, except that there are no lugs and therefore the frame's pieces need to interlock with very close tolerances. The Schwinns howeee lists above were all fillet brazed, as were the Sting BMX bikes, and some one-off prototypes that led to electroforged versions down the line.
Fillet brazing is a bicycle construction technique that has been used by makers other than Schwinn for a very long time.
Most Chicago Schwinns were electroforged, which yields a smooth, seamless look similar to fillet brazing but is done by machine and requires very thick-walled tubing to withstand the process.... which is why a Varsity weighs as much as my mom. Paramounts were lugged. Most of the Taiwan "Schwinns" were either lugged or TIG welded. Chinese Schwinns are almost always TIG'd.
This, of course, is why I'd love to see a close-up view of a '95 Phantom. By the 1990s, there were very few Schwinns that interest me even a little bit, but I'm curious about this one b/c the original retail was so danged high, and many "articles" on the web suggest they were made in the USA. I'll readily admit that I'm not sure, which is probably why I'm curious.