Canon Publishes EF 400L IS III / 600L IS III Developers' Interview

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 | Adorama | Amazon | Wex
Canon EF 400mm f/2.8L IS III USM Lens – B&H | Adorama | Amazon | Wex

Another resource you may enjoy is below, a video by Rudy Winston that accompanied the 400L IS III and 600L IS III's announcement.