The "full" gimp documentation here: https://www.gimp.org/docs/python/index.html
It was last updated in 2006 - long before the 2.10.X update. It is a good resource for covering the basics - but it likely will not tell you why certain plug-ins in the Python Procedure Browser aren't working as expected
Full gimp document here: https://developer.gimp.org/api/2.0/libgimp/index.html Up to date as of 2.8.X
The objects within Gimp have "members" and "methods". Members are aspects about the object, while methods are actions that can be taken upon the object.
The image object represents an open image.
image.active_layer is the active layer of the image.
image.layers is a list of the layers of the image.
image.add_layer(layer, position) adds layer to image in the position defined.
image.resize(width, height, x, y) Resizes the image to size (width, height) and places the old contents at position (x,y).
There are also "Channel" and "Layer" objects with their own members and methods.
To be explored in the future. Used for persistent storage while GIMP is open.
Units expects INT32 parameter, despite the units being described as strings in the GIMP interface. Presumably use the following: { PIXELS (0), INCHES (1), etc.. }
Finding x and y positions of a layer the language used by gimp and pythonfu is "offsets", because you are looking for the numbers offset from the 0,0 base coordinate. Search for plugins using the keyword "offset" to find what you're looking for.
Position does not mean x and y at all. The position refers to the order of the layers and the relative position of the layer in that order.