My experience working at Facebook was that the "working at scale" is nowhere near as glamorous as it sounds, especially when it comes to scaling build systems and monorepos.

As a company, you have to invest immense resources in evolving the infrastructure and it is a never-ending task: the more capacity you build, the more your organization will grow in ways that tax that capacity. The goal posts are ever-receding.

As an individual developer, all that fancy distributed infrastructure becomes a barrier that must be laboriously overcome at every step. Your expectations about what constitutes a "fast" operation get gradually distorted. Working on "small" side-projects (a few thousand files or less) feels effortless in comparison.

Not to say you shouldn't seek out the "big tech" experience; just that it is not all roses.