What does last part of line means?

#1

Sum of [1, 0, 2, 9] and [3, 4, 5, 6, 7] is [3, 5, 5, 9, 6] and the first digit represents carry over , if any (0 here ) .

What first digit is it talking about?

#2

first digit represents the carry over from adding the last digits

#3

can you give an example for better understanding?

#4

in case of
1
9
1
9
answer should be 1, 8 1 here is carry over from adding 9 and 9