They are soldered in, you drill a whole through the kettle smaller than the fitting then use a tool to pull it through creating a mechanical joint then solder that joint. It is quite easy to do if you are familiar with soldering.
https://www.brewhardware.com/product_p/ptbulkheadmf.htm
There are some more pictures and directions on there if you'd like to look.
For the elements, I used this kit(
https://www.brewhardware.com/product_p/etc4s.htm) and soldered the fitting on. If you wanted to solder you could use this(same fitting on my kettles)
https://www.brewhardware.com/product_p/tc15fsf.htm Otherwise, yes 1.5" triclover ferrule would work if you welded in.
Yes, the elements are bare, Stainless Camco 02955. The elements are screwed into those pods above and can be unscrewed(along with the ground and (2) 120v leads).