About OpenGL, here’s a long post outlining some of the problems: https://softwareengineering.stackexchange.com/a/88055/25371 While not directly applicable to the Pi where there’s no Direct3D, the GL issues are pretty much cross-platform.

Vulkan fixes that by stripping these problematic higher-level abstraction layers from the GL. Instead, Vulkan provides low-level access to the GPU hardware.

The triangles are still there usually, but the Vulkan has no GL-like high-level APIs to render them. Instead, in Vulkan we create and update GPU buffers and textures with the source data, create pipeline states (an object containing shaders, input layout, alpha blending, and quite a few other relevant parameters), and finally submit draw calls rendering triangles from buffers, using some pipeline state.