STORY 9: "Flow and Reflow"

What are you talking about?
Reflow soldering is the method of applying solder to a pattern with any method.

I cannot imagine the method to do it?

You ask a question about everything. Stop asking questions until I explain it to you!
For example, one method is preliminary solder plating, or placing the solder that has been formed into a shape suitable for the purpose of use, such as disk, washer and global shape onto flux. Another method is solder paste printing, or pasting a mixture of flux and solder powder onto a pattern.
Generally, the solder paste printing method is used.
After applying solder onto a pattern, put a target component onto it. Then, the solder is melted with a certain heat source to solder the component.
This method is classified into infrared reflow soldering, laser reflow soldering and so on, depending on the type of heat source. It can be illustrated like this:

It looks like a pizza.

Ha-ha-ha-ha-ha (Laughing) It looks like a pizza, as you say.

But, soldering irons seem to be unnecessary now, because such convenient methods are available.

No, that's not always true.