forRoot creates a module that contains all the directives, the given routes, and the router service itself
forRoot创建一个包含所有指令、给定路由和路由器服务本身的模块

forChild creates a module that contains all the directives and the given routes, but does not include the router service.
forChild创建一个包含所有指令和给定路由的模块，但不包括路由器服务。

.forRoot(routes)是父亲，.forChild(routes)是孩子；
一家(一个项目)中只能有一个父亲(forRoot)，但可以有多个孩子(forChild)

参考链接：https://blog.csdn.net/i042416/article/details/119002384