Whether or not it changes is one thing, but the point being: it's not a guaranteed aspect of proprietary software, and macOS doesn't do what Windows 10 does.
Also: yes, you can look. You can monitor software's network traffic; the data might be encrypted, but you'll know that something is going out when you perform certain tasks or leave the OS running. There's also the question of theory versus practice. Sure, open source software lets you inspect code, but how many people actually do that? When are Jane and John Typical User going to examine code with a fine-tooth comb to see if Apple is reading their contact lists? The answer, of course, is "never." It's completely unreasonable to expect an everyday user to learn code thoroughly enough that they can make sense of network data... and security researchers don't need open source code to spot suspicious data transfers.