How to merge a appended object?

lets say i create a body in project 1 and in project 2 the head, how do i merge the head to the body?

Ctrl-J (join)

Option 1:
Append the geometry part (File > append > Object) from the other blend.file
Position it as desired with the other part.
Select both geometries --> Ctrl J (joins the two into one object)
Delete eventual undesired faces

For making the transition, you either select individual edges and make Faces (press F),stitching them together one by one.
Or select upper and lower open edge border, then: Mesh --> Edges -->BridgeEdgeLoops (refine in the T-Panel: Nr of Cuts, Twist, Merge Factor,…)

Option 2:
Boolean modifier to first object, then add the second one in the object field --> Union
Apply the modifier in order to get an editable mesh and delete the obsolete operation object.
This only makes sense, when the two geo’s are overlapping…

Either way needs some refining (loop cuts, knife tool, etc.) and vertex pushing for a smooth transition.

thank you ! you guys are the best!