This paper has some nice pictures https://arxiv.org/pdf/1310.7866.pdf, and it’s got Hartmut Ruhl and Amitava Bhattacharjee on the author list. They know their stuff. It covers the basics of particle shapes and mapping quantities from the grid to particle and vice-versa.
All the gory details are in the Particle Simulation Code manual - I can’t seem to google a copy of it.
On the Maxwell solver side there is https://github.com/wsshin/MaxwellFDM.jl, which appears to be under development. I know that the Julia FEM package is crying out for developers, and really feel as though that should be the ultimate goal.