If you move photos into albums does a copy remain in the central library too?

There is only ever the one copy in the central library. Albums do not hold a separate copy.

An album is essentially just a list of items that are in that album. When you view an album, photos just shows items in the list viewing from the library.

So all photos always appear in the central library regardless of whether they are also shown in 1 or multiple albums.

Yes, but it's not a copy.

Every photo is in the Library. Albums contain references to the Library. So a photo can be in any number of albums but not take up any extra space. It's all the same photo.