You can create/or link an existing part to the collection.
To create a new part for Collection, click on “Part & Fitments” and show the screen as the below image.
Here, you can switch the button, after that you will see the collection dropdown.
The dropdown shows all the collections you have added. You can select the collections where you want to add your part and click on Add Parts. It will redirect to your page.
When you click on the highlighted “+” sign button redirects to the add collection page.
Once you add a part with collections, the below page should be shown to you.
Here you can see the part details and fitments for the part. You can modify the fitments of that collection by clicking "Modify Vehicles" the collection