The cheap European stuff was made
"on The Hearth"
The lugs would be in the furnace until red hot, the plain "gaspipe" tube ends held in the heat until likewise, then pushed into the lug along with (maybe) some flux and brass, which would melt and flow from the heat.
Just a few seconds per joint.
The whole main frame would be assembled like this, almost no need for movement from the assembler.
The back end would have the stays crimped onto a stamped dropout, 3mm? The dropout jaw slope allowed for inaccuracy.
The top of the seatstays squashed flat and back in the hearth with the front end for another blob of braze.
The whole thing put onto a flat table and bent into "alignment"
Job done.
I heard some people could make a dozen or more a day - dirty smelly work.