Questions about example sentences with, and the definition and usage of "Weld"

A: It’s a common type of flange. You can google and read about it.

A: Lest is archaic or very formal. You could use "unless" or "in case" or "so that" instead. Welds are joins in metal that are made by a torch. It's being used as a metaphor here.

A: Welding is using heat to permanently attach one piece of metal to another, so it means “I am attached to you” or “I’m stuck to you, permanently.”

A: To weld is to join two metal objects by melting both so that they combine.
To solder is to join two objects (usually wires) by melting a metal (the solder) and then joining as the solder cools and becomes solid. The wires are not melted.

The two pieces of steel are welded together.
He welded the two pieces of steel together.
The electrician joined the two wires by soldering them together.
He heated the solder and then applied it to the two wires.

A: Weld is for big pieces of metal.
Solder is for (very) small pieces of metal. Soldering is usually used for electronics.

A: *what do you call it when you have to...

I don't understand the situation, could you give more details?