How does FileType lazy_handler_event works?

[kls1991]

Below is my attempt, language server will not attach to the first lua buffer I open, seems require("lspconfig").lua_ls.setup {} happens too late:

vim.api.nvim_create_autocmd("FileType", {
  pattern = "lua",
  once = true,
  callback = function()
    require("neodev").setup {}
    require("lspconfig").lua_ls.setup {}
  end,
})

Typical lazy spec, work as expected:

{
  "neovim/nvim-lspconfig",
  ft = "lua",
  config = function()
    require("lspconfig").lua_ls.setup ()
  end,
}

I eagerlly wonder how Lazy implement this, Lazy seems to make filetype autocmds happens earlier? Is that possible?
What does lazy_handler_event group's Filetype autocmds's callback function look like?
I find a file lazy\core\handler\event.lua, but it's too hard for me... Read it several times but still don't get how Lazy does this.
Sorry for my bad English :)