As Alexander points out in the comments, list addition is concatenation.
a = [1,2,3,4]
b = [10] + a # N.B. that this is NOT `10 + a`
# [10, 1, 2, 3, 4]
You can also use list.extend
a = [1,2,3,4]
b = [10]
b.extend(a)
# b is [10, 1, 2, 3, 4]
and newer versions of Python allow you to (ab)use the splat (*) operator.
b = [10, *a]
# [10, 1, 2, 3, 4]
Your choice may reflect a need to mutate (or not mutate) an existing list, though.
a = [1,2,3,4]
b = [10]
DONTCHANGE = b
b = b + a # (or b += a)
# DONTCHANGE stays [10]
# b is assigned to the new list [10, 1, 2, 3, 4]
b = [*b, *a]
# same as above
b.extend(a)
# DONTCHANGE is now [10, 1, 2, 3, 4]! Uh oh!
# b is too, of course...