This depends a lot on what version of Unity/URP you’re using, and is largely undocumented. Likely any blogs/tutorials you find will be slightly outdated. I did a few custom shaders for 2021 (URP12) so I can answer any specific questions you have. The G-Buffer format itself is fairly straightforward; the real challenge will be shadows, and all the meta/editor picking/depthonly/etc passes.
At this point, I’m just using Better Shaders for anything that doesn’t need custom lighting, which is basically the same as surface shaders. Supposably, block shaders will be around someday. Likely out of beta in the 2025+ cycle or beyond.
If you need custom lighting (eg hair, glass, water, skin, scales, oil, water, crystal, eyes, wool, fabric, etc)… you’re on your own. Most of that won’t work in deferred anyways.