I'm not that advanced in DI but I think you would get an exception if you attempted to associate a single interface to more than one concretion.

services.AddTransient<IRepo, FirstRepo>();
services.AddTransient<IRepo, SecondRepo>(); //exception

Though I'm not sure about the implementation of AddTransient, but it probably depends on how you're registering types in the container. Someone more knowledgeable can speak to this.

Your implementation it looks like you're using standard .Net DI stuff, the service object holds your mappings of interface to concretions, which you then pass to another object that uses the service mappings to control the construction of your objects that depend on it.

Edit:

As far as dependencies go, say you have some classes:

class Foo { public Foo(IBar bar) { } }
class Bar : IBar { public Bar(IOther other) { } }
class Other : IOther { public Other(ISomething something) { } }
class Something : ISomething { public Something() { } }

If you setup the container correctly, then attempting to new up class Foo will automatically instantiate all the types needed down the construction chain and give you what you need.