There are millions of servers out there.
Most of them are not accessible to the general public.
Most of them serve the needs of individual companies, government agencies, etc., accessible to the people who work at those places, so they can do their jobs.
There are probably thousands of different kinds of servers, running on various different kinds of operating systems.
You may not be aware of this, but the World Wide Web, which uses the http;//www. prefix ... that was invented perhaps 15 years ago ... but the Internet existed for 20-30 years before that, using other kinds of connections, many of which are still out there, can be found by Internet search engines.
The key issues include
* mission critical ... this is not in most people's vocabulary, but for some places like hospitals, air travel, etc. it is just not acceptable for things to be breaking down regularly, or for criminals to be able to access and change anything they please
* efficiency of access to gazillion records & data
* format of data, interchangeability
* security & privacy ... this is another concept that is missing from most people vocabulary ... computer engineers know how to make secure systems that guarantee privacy, but no one wants to spend the money to get them ... everyone wants the cheapest, and then to complain about the consequences of that, but not do anything to fix it