My understanding is that previously it was too costly to do this because we had to do it in each project individually. Now, however, we can do it all at once, globally (while preserving the ability for product-team developers to run formatting and linting locally within their individual projects).
"yarn checkFormat" can be executed from the top-level "modules/" folder and will cover all .JS and .SCSS in the repo ("all", relatively speaking, because there is some amount of blacklisting and whitelisting going on in the background). It takes about 52 seconds to on my laptop.
It will exit with a non-zero exit code if either formatting or (non-presentational) linting problems are found (but note: it always checks both and doesn't abort even if the first one fails, in order to maximize signal). The console output describes the issues, but in practice most developers will run "yarn format" to auto-fix the issues, or "yarn checkFormat" to see them locally for themselves.