First of all, don’t worry. It is very common in CS that you may be flagged atleast once. But as long as you know what you did and can proper explain it, you should be good. Talking about the similarity part, the MOSS or whatever tool they are using, checks the code for compilation style, excluding the syntax style or the comments, so if you think that the other person did it differently based on the syntax, no MOSS doesn’t look at that. I am a TA so I have seen how it works, and normally also seen that the instructor generally does not care if there is a 20% similarity or so, if that is something like variable declaration loop etc., but if there is other similarity as well, then the instructor usually flags you. But all the best, hopefully it is sorted for you