Who hosts the website hosting companies? Can't we do like those companies and host our sites from your own PC?

Technically, you can host a website on your PC. However, the reason we use web hosting companies is because they have much faster connections to the internet than we do at home. Also, web servers stay online all the time, unlike our home computers, which we need to turn off and reboot more often. It's very noticeable when you try to visit a website on a home-based network, as the site loads slowly (especially if more than one person is trying to use the website at the same time) and sometimes not at all. If you have a dedicated server in your home and a high-tier internet connection that's only used for your web server (that is, nobody in the house is streaming TV shows, playing games, etc.), you can definitely host your own website without paying a web host.