Ken

I have used water hammer arrestors and copper pipe for water service/caps. Even the hard copper can be squeezed flat and soldered if you want to do that.

Rookie, I used silver solder, but any lead free solder would work. I think the key is to clean the metal well and use good flux and the right flame. Practice!

Yes the tubes drop down into the flue, but the solder doesn't melt because the tubes are full of sap which limits the temp to the boiling point, just like in other soldered pans. Don't run them dry!