why when i make two similar lists in python they are linked together code example

Example 1: append two list of number to one python

listone = [1,2,3]
listtwo = [4,5,6]
mergedlist = []
mergedlist.extend(listone)
mergedlist.extend(listtwo)

Example 2: combine to lists python

listone = [1,2,3]
listtwo = [4,5,6]

joinedlist = listone + listtwo