Copy-paste and Send To are both quite broad concepts.
Cut, copy and paste operations are related to the clipboard - a special area where various items can be stored temporarily after being cut or copied.
What Cut and Copy operations actually do depends on what you're applying them to. For example in Notepad, if you cut some text, it's instantly removed from the Notepad's window and put into clipboard. Copying it will create a copy in the clipboard, but without removing the original text. The text in the clipboard will be marked as plain text. You can paste it as many times as you want, no matter if you've cut or copied it. The clipboard looks like this:
Plain text: The quick brown fox jumps over the lazy dog.
In Word, if you cut or copy something, it will land in the clipboard too, but will be marked as image, table, formatted text or whatever it is.
A formatted text: The quick brown fox jumps over the lazy dog.
Pasting content from Word into Notepad will remove the formatting, making it plain text. When doing the reverse, Word will ask what style you want to apply - plain text doesn't have formatting, but everything in Word document does.
Similar rules apply to pictures. You can cut them once or copy them, then paste infinite amount of times. Picture is always a picture, so there are no formatting problems. You can't paste a picture into Notepad (because it's not plain text), but you can paste it into Word (formatting will be applied automatically).
Files also work in this manner, but as you've noticed if you cut a file, then you can paste it only once (which was not the case in previous examples). The cause of this behavior is actually pretty simple.
Let's say you have 256 MB of RAM (I know it's very unlikely today, but let's assume that for the sake of this example). You want to copy a file to another folder, but the file is about 1 GB - four times more than your total memory, not speaking of the free memory. Now, it won't work like that. What's more, even if the file would fit in RAM, moving it this way would be dangerous, because clipboard is lost after shutdown, so a power outage could destroy some important files if it happens amidst copy-paste procedure.
What actually happens when you cut or copy a file is its path is put into clipboard with an optional annotation to delete the original after pasting it somewhere else. So you don't have entire file in the clipboard, only its path and some details on what to do with it:
A file: C:\MyFile.txt, don't remove the original after pasting it somewhere.
or if you cut it instead of copying:
A file: D:\Videos\BigBuckBunny.mp4, remove the original after pasting it somewhere.
When you paste a file that was cut, the path from the clipboard is resolved and the file is moved or copied. There are two major consequences: first, that's why you can paste cut files only once - the path is still in the clipboard, but it doesn't exist on the hard drive anymore. Second, the cut file doesn't disappear instantly, but when it's pasted somewhere else - if you modify it in the meantime, the modified file will be moved.
Now, Send To isn't much less complicated. Just as applications can put different types of data in the clipboard, Send To can have many kinds of targets. It's implemented in a dirty and hacky way, but it would be hard to do it the other way, preserving its functionality.
Send To targets are actually files in some directory, you should see them by typing this in the Start menu and pressing Enter:
You'll notice that some of these files are shortcuts and some are not. About shortcuts, it's simple - when a file is sent to it through the context menu, it's passed as an argument to the shortcut's target. All further processing is left to that application, Windows isn't involved anymore.
About these files that aren't shortcuts - these are kind of special files that are handled by Explorer (the application that's responsible for showing desktop and Start menu). When you choose Send To -> Documents, Explorer will handle your request and create a copy of the file. Details of this operation depend on how Explorer handles it, it's probably just a simple file copy operation. (I mean just pure path-path copy, no clipboard involved - programs don't have to use clipboard if you know exactly where to copy a file, clipboard is just a temporary place for the source file's path if you don't know the target yet)