You are correct. If you already have the app, it doesn't require a live connection. If you don't have the app, you'll need to download and install a 50MB file.
Apple does produce lots of open source - https://opensource.apple.com/
Microsoft, famously, publishes a huge amount of open source on GitHub - https://github.com/microsoft
I think it is entirely reasonable to say emergency services should be free to use (gratis). We don't pay for making 999 / 112 calls.
It is also prudent that safety critical software should be free (libre). It is important that we can examine the code to make sure it doesn't contain any nasty surprises.