It looks like the element itself sits inside a casting (presumably the mag/aluminium alloy) which itself sits on a disk of the same material which is screwed to the stainless base.
It's not entirely clear is the screws in the picture are holding the element housing to the base or are there for some other purpose, but that is certainly one way to do it.
You can see slightly darker grey area around the joint which I suspect may be thermal paste/adhesive of some description to ensure good thermal conductivity between the element casing and the stainless steel.
Thermal epoxies ect are typically good for for up to around 300 deg C which should be plenty for a kettle as the boiling water will never get hotter than 100 deg C and acts as a good heat sink, although of course the element itself will need to get somewhat hotter than this.
It's not really possible to weld aluminium to stainless steel (certainly not with any general purpose production process) , they can be soldered together with the right filler and flux, although these tend to be relatively low strength and low temperate and so may not be that much better than a combination of mechanical fixings and thermal paste.
The thermal paste/adhesive is important because unless you have a very, very good mechanical fit between the element and the stainless steel there will be a significant gap over much of the contact area, greatly reducing the rate of heat transfer. The thermal paste is very conductive (usually aluminum or sliver particles in some semi-fluid medium) which fills the gaps and creates direct, conductive contact over the entire surface.
There are processes which involve bimetallic interface inserts to allow welding of steels to aluminium alloys but these definitely fall under the category of 'advanced' welding.
One of the reasons for using the aluminium casting is that the thermal conductivity of stainless steel is fairly poor so you need a heat spreader made of a conductive material to distribute the relatively high temperature of the heating element evenly across the stainless steel to water interface to get the best heat transfer rate from the element to the water.
Using a copper housing for the heating element would give you more options for higher temperature bonds as it's a lot easier to weld or braze copper to stainless than aluminium to stainless.