Recommended and mentioned products
ZoneMinder is a free and open-source piece of security software that can be used at home or in your business. The software puts you in control of all of your data, and it is exceptionally easy to setup. Read more about ZoneMinder.
I use Zoneminder currently with ~$50 PoE cameras (I have an assortment based on what was on sale but most are 4K with audio). I block the MAC addresses of the cameras so they have no internet access at all. You could accomplish this also by putting them on their on VLAN that doesn't have access but I didn't have a good way to do that when I first set it up. My plan is to switch to a Reolink NVR in the near future...
Blue Iris is a high end security monitoring system that lets you view and control the feeds from all the cameras at your home or place of business.
I run Blueiris (https://blueirissoftware.com/). The beauty is you can run a mix of cameras, so I have decent Hikvision ones outside and whatever cheap PTZ I find at a good price inside.