Good question with a simple answer.
The layout is structured as per the menu ie, “Chassis Assemble” then “Fit Body”.
When you hover over “Fit Body” you will see the full sequence such as:
Fit Body Shell
Fit Trunk, etc
Now when you start out you will only be able to place one large “Fit Body” station, but as you research and unlock more options you will be able to replace it with the individual components of that activity ( as mentioned in the list above).
The sequence will always be as it is in the menu, unless for example with “Paint” you do not want the “Polish” feature and have not included that in your vehicles specs.
I hope this helps.