Intranet Development with simple functions

Is my understanding of Intranet = essentially password-protected admin website, with credentials only for registered employees, wrong??

I'm confronted with a task where a client wants to have an intranet with really basic functions (file uploads + file storage on the server is the most "complicated stuff"). I've never developed a so-called intranet so far, and the client already has a website.

What I was actually thinking was just adding an "Intranet" or so link to the existing website, which points to a password - protected subsystem / subdomain, or even a single additional page of the existing site, which in turn represents the intranet.

Ready to go, I consulted some tutorials, blogs and guides about intranets, and wondered why they all speak about you NEEDING an intranet software, like Microsoft SharePoint or whatever. Isn't an intranet developed with these essentially the same as a password-protected administration website? Or am I lacking something here? Of course, softwares come with advantages and ready-to-use stuff, but I've developed quite some login systems for now and feel that it is much easier to develop an intranet with very basic features only as elaborated above on my own, instead of choosing + learning the ideal software. In my opinion, developing one on my own would especially make future edits + adaptations much more convenient..?