I downloaded a varsity jacket from sketchfab and am having trouble properly rigging it with my character. When I move the arms, torso parts of the jacket that aren't connected to the arms move also, which can be seen in the following images.
I'm thinking the problem lies within the weight paint data for the jacket.
To assign weight paint data to the jacket, I essentially stole the it from the body mesh by using a Data transfer modifier on the jacket, setting it to vertex data and 'nearest vertex' so the jacket would copy the weight of the nearest body mesh vertex. This worked somewhat, but gave me the problem shown here where I had unwanted parts of the jacket I deforming.
I tried manually weight painting to see if I could fix it at all or even weight paint some parts from scratch, but no luck there. My guess is the data transfer didn't work perfectly as the body mesh was pretty different from the jacket mesh (body is quads and jacket is tris btw) and resulted in some janky weight data, but regardless I'm still not totally sure what the fix is...
My blend file can be found here if you need it, thank you for reading my post and any help is very much appreciated!!