One way that Git scales to the largest monorepos is the sparse-checkout feature, which allows you to focus on a subset of the files. This is supposed to make it feel like you are actually in a small repository, even though you are contributing to a large repository.
There’s only one problem: the Git index is still large in a monorepo, and users can feel it. Until now.
Source: Make your monorepo feel small with Git’s sparse index, an article by Derrick Stolee