So I’m looking at your ‘$ Spent by Category’ tab. How are you getting the totals right now?
I’m going to make assumptions on how your data is laid out. What I think you need is a relation column in the Categories sheet to link the category name to the category name in the individual Items sheet. This will 1. allow you to create an inline list of all the related items from the items sheet, and 2. create a rollup column that uses the relation column to sum the values of all the individual items that match that category. Use the rollup column to display a total value of all related items in the category.