Usually they will ask for feedback, or specify what they are looking for in feedback. If they don’t, then you could just mention what you like about it. “I like the colors and timing” or “I really enjoy the shape language”. Focused compliments are a good type of positive unsolicited feedback. If you have 5 people saying they like your colors, then you know you’re probably doing well in that aspect of your work. If no one compliments you on your timing, or your shapes, etc. that could be an indication of where to focus more of your time and practice. Although that’s not a hard and fast rule; that’s up to you to determine.
As for what to do with feedback, at the very least you should consider it. You can decide if you want to apply it or not. If you asked for feedback and received some, then apply it, even if it’s in a duplicated/v2 version of your work. If it was unsolicited feedback, try and think if what was suggested could make your effect better or not. If you’re unsure if it could improve what you have, then give it a try and see how it looks.