Whe can I add new "LuaSnip" snippets?

Plugins
2

Caveat: I don’t use this plugin (yet)

It sounds like you can either assign the snippet (as a Lua object) directly to the snippets or autosnippets table, or put JSON snippets in a directory and use lazy_load({paths = '...'} as you discovered.

Both versions are shown the sample configuration:

-- Define some snippets
require("luasnip").snippets = {
  -- Snippets for all filetypes
  all = { ... }
  -- Snippets for filetype=python
  python = { ... }
}

-- Load snippets from JSON files (VSCode syntax)
require("luasnip/loaders/from_vscode").lazy_load({
  paths = {
    vim.fn.stdpath('config') .. '/snippets',
  }
})

You could also do something more sophisticated with the paths, like finding all snippets/*.json files anywhere in &runtimepath:

--[[ Split a string into lines. ]]
local function split_lines(txt)
  return vim.fn.split(txt, '\n')
end

--[[ Find files in the runtime path. ]]
local function glob_runtimepath(pattern)
  local runtime_path = vim.fn.escape(vim.o.runtimepath, ' ')
  local search_result = vim.fn.globpath(runtime_path, pattern)
  return split_lines(search_result)
end

require("luasnip/loaders/from_vscode").lazy_load({
  paths = glob_runtimepath('snippets/**/*.json')
})