The file extension for, Adobe After effects project is .aep.
Motion sketch is a path you want your object and picture to drop or move. For motion sketch, click on the window tab in menu bar and select Motion Sketch option from the window bar. Now you can draw the path and make your image move accordingly.
In edit bar, if you select split option you can split your video clip into two sections or layer. So, when you want to do animation separately for each layer or each video clip you have separated, you can do it by using split layer option.