It records in a loop, overwriting the oldest.
I bought the "better" version, higher resolution cameras, 4TB hard drive. It has some really nice controls and features in it. More of the stuff I would expect in a "Professional" NVR system. I've had it running for a few months without a hiccup. I wanted the wired system over wifi, for reliability, and to keep wifi traffic at a minimum. One thing I really like is that to use the remote access, you don't setup an account to give anyone any identifying information.
You mentioned a keyboard on it, I tried, it didn't work, IIRC. I will usually access it with the web interface, you just have to locate the IP address for it, and type that into your browser. It's a bit more user friendly than the mouse and monitor connected to the unit. It doesn't work in Chrome, so I use Internet Explorer.
As for storage space, mine with the higher resolution cameras, only running 2 of them, I have 24/7 recordings since Feb 3, and it's just about to run out of space, and start overwriting.
One thing I love about mine: I have one inside, pointing at a door, and monitoring the dining room/kitchen (kids.) I will pull up the cameras on my iPad, the use Google home to broadcast through the Google Mini, and freak the kids out! It's funny to watch!
I did some research and the system is based on a Chinese system, but at a better price, with the WD name behind it. If I need to add more cameras, I might just buy a complete second system, because it comes out cheaper than 4 cameras!