How can I scan a thing and modify it or scan two things and compare them? I've been asking people about this, not no clue yet.

I want to scan a thing and modify it a little.

For example I have an object with 3 parts and

I want to change the position of the third part relative to the other 2 parts.

For example I have a certain object and I want to modify it a little to see

how the air flow around them would change.

Or I want to scan two similar things and see what's the difference between them

PRECISELY... is there any way to do this?
While I'm not sure what you mean by I want to scan a thing and modify it a little to see its effect. In general, a 3D scan typically results in a series of raw points. You'll need to convert the 3D point cloud into a Mesh before you can really do any solid modeling with it. Here are a few OpenSource programs that you could potentially use:


MeshLab
Blender
CloudCompare


To compare two scans, you could use the raw data in CloudCompare as well.

However, without knowing what your intended use is, it's difficult to tell how to help you. Please consider updating your response with a bit more of your own research so that we may definitively answer your question.