Do you remember what I told you about the internet? If you are new then I would recommend the below article to read first.
The internet is a worldwide network where we can retrieve and send any kind of data over the network from one computer/device to another computer/device.
Over time, those computers which send data became important and they needed to be specially configured so that they can serve data fast and efficiently. Then a special operating system was created which is known as the server operating system (Server OS). There are many server operating systems available out there such as Windows Server, Ubuntu Server, RedHat Server and many more.
Only the operating system makes a computer a server or a normal computer (client). You also can convert your normal computer into a server by installing any Server OS and configuring the necessary steps. If a computer has a server operating system installed, it is known as a server. The server is just an operating system or computer program that serves the resources and data to its user also known as the client. Whenever a client requests any resource from the server, the server uploads that resource and the client download that resource.
One more thing, a computer can not run files online, you can never run files online, if you watch youtube videos, your computer first requests that video from the youtube server, that server first authenticates and then returns the video as a response, youtube server uploads video and your computer downloads that video in the form of cache and saves it into your secondary storage device (Hard Disk or SSD) and then your browser able to run that video. The same process goes for any kind of online surfing.