I spent the last week configuring apache on ubuntu and i got my site running on my router(I could access it from different computers in my house), but I couldn't make it available on the internet. I watched a couple of videos about buying a domain name on godaddy.com and giving godaddy your router's IP address to make your site available on the net, but then I found a free hosting company called Infinityfree and I just upload my website on it via FTP and I never even used Apache. So why use Apache in the first place? Are internet hosting company mandatory in order to make my website available on the internet?

To get to your site, it needs to be hosted somewhere and have an IP address. For a small site, the best option is a web hosting company as you found out. For a larger site with a lot of traffic, this can get expensive so you may want to move up to someone like AWS where you are paying for servers and traffic but running your own customized stack with apache or node.js. Only someone very big or doing it for fun runs their own servers. Home ISP providers may not allow you to run servers; check your contract.

Any of these will also need a DNS provider to host their domain name(s) and point to the IP. That can be provided by the web host, or by a separate DNS provider. Your home IP address is likely not static, but there are name services that will provide dynamic DNS to home IPs.

In the early days there weren't as many options, so I used to run servers for DNS, web and email for a small company. I outsourced to a web hosting company 20 years ago. The economics made it an easy choice. I kept the DNS provider separate in case I ever got into a dispute with the web host company.