I would assume you have to use stock. Excel/Office ships with a VBA IDE that you get to with alt-F11 (and I think by viewing macro source through the menus.)
There's a macro recorder, but the code it generates isn't meant to be efficient or readable.
VBA is easy, you can just google it. It's very bad, but once you know it well, and if you stick to a functional style, you can do what you want. For Excel, get to know Range objects well, and to wring some polymorphism out of VBA, learn interfaces.
I've seen basically Mario in VBA in Excel, so I know it's real. "Show formulas" && [VBA, JS] vs an app with test coverage.
Are unit and/or functional automated tests possible with VBA; for software quality and data quality?
GSheets has Apps Script; which is JS, so standard JS patterns work. It looks like Excel online can run but cannot edit VBA macros; and Excel online has a JS API, too.
