> I work at a small startup

How does a “small startup” end up with a multi million line “legacy” codebase? Something not mathing

"startup" and "legacy codebase" are diametrically opposed concepts.

And if you're saying (based on your other comments) that a 6 month window is enough to create a legacy codebase...that indicates a serious lack of experience or understanding as to what a legacy codebase is, or why they exist.