I understand how the simple list comprehension works eg.:
[x*2 for x in range(5)] # returns [0,2,4,6,8]
and also I understand how the nested
The above answers will suffice for your question but I wanted to provide you with a list comprehension solution for reference (seeing as that was your initial code and what you're trying to understand).
Assuming the length of both lists are the same, you could do:
[l1[i] + l2[i] for i in range(0, len(l1))]