I don’t know what Hosoda said about about mixing CG and hand drawn animation in Summer Wars, specifically but I do recall he had some things to say about Mirai, which I had seen before - another good Hosoda film imo

https://www.awn.com/animationworld/merging-hand-drawn-tradition-cg-artistry-mamoru-hosodas-mirai

He used CG in Belle for the cyberspace scenes and the traditional hand drawn look for the real world scenes. Haven’t seen Summer Wars yet but the preview seems to indicate it follows that pattern too.