Comment by mindcrash

Proton is mainly a co-effort between in-house developers at Valve (with support on specific parts from contractors like Igalia), developers at CodeWeavers and the wider community.

For contextual, super specific, super specialized work (e.g. SCX-LAVD, the DirectX-to-Vulkan and OpenGL-to-Vulkan translation layers in Proton, and most of the graphics driver work required to make games run on the upcoming ARM based Steam Frame) they like to subcontract work to orgs like Igalia but that's about it.