Why last lines len items showing output 5 result😐..can anyone plz clear

Question

x=5
y=20
print(x+y)
if x>=y:
print(y+5)
elif y<=25:
print (x+203)
z=[x,1,3,5,7]
z[2]=(10)
print(z[0])
a=[1,2,8,5,z,"string"]
print([2])
print (a[4])
m=z*3
print (m+z)
my =[x,y,z,[1,2,3]]
my .append(9)
print(my)
print(len(my))

Answer 1

x
y
z
[1,2,3]
9
-------------
5 items