Ok, so do you coat the end of the tube with a little bit of solder, hit the lug with the flame, and the solder melts & forms the bond?
I assume you also need to clean up the lug after that? Or do you just paint over it.
(FYI I've done a little bit of brazing in my time, nothing remotely like building a frame though.)
To begin with, the tube ends are mitered so that there's no gap between the tube end and the tube you're joining it to. You use specialized fluxes on the tube end and inside lug surfaces, then heat the lug so the heat draws the melting silver solder into the area between the inside of the lug and the outside of the tube. You do spend significant time and effort cleaning up the lugged joint after you've finished with the torch, and frames that are to be chrome plated require more careful cleanup than painted frames since any blemishes show up through the plating, but paint covers up a sloppy cleanup.