What is the difference between the Recent and Library folder in Photos

Similar, but not quite the same. The details seem to change a bit with OS updates, but Library, Recents, and Imports are all provided by Apple for specific purposes.

• Library contains all the pictures in your Photos Library in the order they were captured, according to the pictures' metadata.
• Recents also contains all the pictures in the Photos library, but they are sorted by the date added, not the date taken, and it seems that certain manipulations may move pictures from their original spots to more recent ones. Duplicating a picture, for instance, results in a picture that is exactly the same in every way, except that it appears later in Recents.
• Imports is sorted like Recents, as you say, except that some pictures may enter Photos by some method that doesn't go through the usual Import routine (maybe through a 3rd party app, like Power Photos,) and they don't appear in the Imports folder. Duplicates don't show in Imports. So, unlike the other two folders, Imports does not show all your pictures.

If your only source of pictures is a single iPhone, then all these will look pretty much alike. But if you scan old pictures, or import pictures from a camera's SD card, and such things as that then the lists will look more different.

Well, any of this could change with a new OS, so then we will both know nothing.