I'd agree with others, I don't think you need a custom "app", you just need to upgrade your spreadsheet skills. That's probably the easier path here.

Theoretically you could make your components variables and just point your sheets doing the calculations at those variables. So you just change value of your variable, and your sheet re-does the calculation.

The only reason I'd recommend doing something custom is if you're building a client-facing form. But even then, a simple Google Form that points at Google Sheets would work.

You can create a drop down box in excel https://support.microsoft.com/en-us/office/create-a-drop-down-list-7693307a-59ef-400a-b769-c5402dce407b

Create a master list of component, price etc

Reference this master list in your other sheets

There's no need to complicate it like that. You just need to learn to use a spreadsheet properly.

If you really really wanted a web app for this you could probably use appsheet, but it's backend would be google sheets so you'll still need to learn a spreadsheet program.

That sounds like the kind of stuff we make at $dayjob (that's for the building trade, where they often have complex spreadsheets and going to an app that calculates everything down to the number of screws for them is a huge benefit).

You could probably still do it with a spreadsheet, just have parts list add/remove fan out into the cutting list and update automatically. I imagine it'd get quite ugly, but doable. If you want to do optimisation (buy 1 5 foot length and cut rather than 2 2 foot lengths, because it's cheaper, for example) it gets even uglier and at that point a bespoke app becomes more sensible.

As others have said, this is something you could probably do in a spreadsheet, but I would expect that to become unwieldy as your business grows and/or your BOMs start getting bigger. You might want to look into some sort of Enterprise Resource Planning (ERP) system. There are some open source options that you could run yourself. However, ERP systems are comprehensive suites of tools for operating a business, which might be a lot more than what you need right now.