I have all of the parts on the left wing connected with Weld Constraints to a main part, which that main part is connected with a Weld to the holder part. (It doesn’t matter if I used a Weld or Motor6D, it’s the same result)
This sounds like an issue with C0 and C1. Those two values represent the offset of the objects. If you don’t set one of them or if you set both of them to the same value, they will be in the same position. What you need to do is set the value of the part you want offset. In this case I believe that would be the “main part.” You would leave the value for the “holder part” alone.
This differs from WeldConstraints, which keep the position of the objects relative to what they were.
Yes you attach it to the holder part. That is done by the Part0, Part1 values. At that point they are connected. C0 and C1 are different values, which you need to set through a script, and you can think of them as the distance between the parts.