Here you go, for starters CTO’s presentation: https://media.ccc.de/v/Camp2019-10238-a_mobile_phone_that_respects_your_freedom
This thread: Breaking Ground – Purism
And: Understandig the giant task Purism has achieved
Plus: Librem 5 — Development Chronology
(Edit, about waterproofing: https://forums.puri.sm/t/librem-5-phone-quality-feature-waterproofing)