Canon has published a very detailed Devlopers' Interview for the EF 400mm f/2.8L IS III USM & EF 600mm f/4L IS III USM Lenses
(PDF, 19.6 MB).
In the interview, the developers discuss their motivation for significantly reducing the lens' weight and the implementation of their solutions, such as rethinking optical designs and employing the use of new glass materials and carbon reinforced magnesium alloy.
Canon EF 600mm f/4L IS III USM Lens – B&H
Canon EF 400mm f/2.8L IS III USM Lens – B&H
Another resource you may enjoy is below, a video by Rudy Winston that accompanied the 400L IS III and 600L IS III's announcement.