Not OP, but it looks like they are using texlab for completion: https://github.com/neovim/nvim-lspconfig/blob/master/doc/server_configurations.md#texlab

Apart from that there is https://github.com/lervag/vimtex with file type support, document compilation, etc. For grammar checking I use https://github.com/neovim/nvim-lspconfig/blob/master/doc/server_configurations.md#ltex and for snippets https://github.com/L3MON4D3/LuaSnip. See https://github.com/iurimateus/luasnip-latex-snippets.nvim for latex snippets and what you can do with them.