The editor itself is in C; I don't think there's any "learning materials" available for the (Neo)Vim internals specifically, so you just gotta learn C. Overall, it's not that hard IMO: I'm not a very experienced C programmer, and I managed to get some patches into Vim. Neovim also uses Lua in various parts for its internals, but I'm not very familiar with Neovim.
If you want to work on things like language support (syntax files, some completions) then most of that is in VimScript; "Learn VimScript the hard way" is a good place to get started.
The editor itself is in C; I don't think there's any "learning materials" available for the (Neo)Vim internals specifically, so you just gotta learn C. Overall, it's not that hard IMO: I'm not a very experienced C programmer, and I managed to get some patches into Vim. Neovim also uses Lua in various parts for its internals, but I'm not very familiar with Neovim.
If you want to work on things like language support (syntax files, some completions) then most of that is in VimScript; "Learn VimScript the hard way" is a good place to get started.