I don’t believe I was able to summarize the issue properly on the title, so I apologize for that, however here’s a little more descriptive explanation with photos.
oh i see what you mean now. There are 2 ways of doing this. The thing is, if you resize the part, it resizes from both sides, so it would not be like in the video with a simple code, so what i recommend doing, is changing black parts position, rather than resizing white part
blackPart.Position = Vector3.new(whitePart.Size.X/2, --whatever Y you want, whatever Z you want)
This would move the black part to the edge of the white part. Note that the black part has to be a child of the white part, so that it moves relative to it.
If this solution doesnt suit you, doing the other way would be much harder and unnecessarily complicated, but we can still work it out.