Seeking clarification on how ports, port forwarding and changing port numbers works.

Hello! I have finally decided to broaden my knowledge on networking and I found that I have still not grasped the basic concepts of ports and their security advantages. The following are the questions I still have even though I have read up numerous material on internet ports.

1. If port numbers for certain services (like 5000/5001) are standard and easily found out, why are “hackers” not able to send data through to unsecured ports like port 5000 and get access to the networks? Is it not that simple? Isn’t that what port-sniffer tools are for?
2. What’s the difference between [port 80 and 5001] and [port 5001 and 443]?
3. What is the practical benefit of forwarding [port 5000 to 80 / port 5001 to 443] or vice versa (I’m not sure if it even works vice versa or if that is the correct forwarding order).
4. Why do people change the port number even after port forwarding is completed?
5. If one changes the port number, will this not lead to some level of confusion since certain services run using those ports?
6. Will a port sniffer tool be able to find open ports if port forwarding and then port number change is done?

I’m definitely still a noob and I will appreciate any information that will help me understand these concepts.