Workflow product design solidworks

I have a question about solidworks in general. What is the workflow when designing a product with multiple parts? Do you make a new file for each part and assemble them in a assembly or do you do a multibody part in one file and save the bodies as seperate files afterwards and then assemble them. In the first instance how do calcutate dimensions, shapes and interferances with different parts when you are are only modeling the single part.
3 Answers

It really depends on the project. The two methods you described are both valid, and there is a third, which is a blend of the two you mentioned.
If a product will have many parts that need to fit tightly with their neighbors (a computer mouse for example), it may be best to create the mouse as a single part and then split it into smaller pieces. But, there may be pieces that you'd only add at the assembly level (i.e. batteries) since they are a common part, or it is supplied by someone else.
The third method is to create a new part directly within the assembly. By doing this, you are able to design new pieces that fit perfectly into the assembly by using other parts as references.
Check some resources for the differences between "bottom up" and "top down" design methodologies. There are many rules and guidelines to follow in both situations to prevent a situation where parts become "broken", or have their shape suddenly change.

Thank you, great advice. I think I will you the combination of the two for my workflow. Additional question. When you save the bodies from the original file and assemble them. When you make adjustments do you do this in the original file or the seperate save body part file?

You can do both.
If you make additional changes to the original file after saving off the bodies, then you MUST make sure that the features you add are ABOVE the Save Bodies feature in the feature tree, otherwise the changes will never propagate to the separate part files.
You can also open the separate part files and add new features to them.
Personally, I'd keep as much work in the original file as possible. I find that adding features into the separate part files can lead to some frustration if changes need to be made later. It can be difficult to know which features are applied where.