Its sanitary welding, where you use a back purge and a few different techniques. Your boil kettle does not need to be welded this way.
If you are modifying a fermenter or anything that touches your beer after it is cooled then it is recommended to use sanitary welding techniques.
Denny Conn - “The best beer possible with the least effort possible while having the most fun possible!”
Me - "Gun Control Means Using Both Hands."