This is one part of a _very_ big answer, but Bazel (internally Blaze) lets you do reverse dependency querying [0] with the query language, i.e. "Bazel, give me the list of targets that depend on this target that I've just modified"

$ bazel query 'rdeps(//foo/my:target, //...)'

Of course, this query in the monorepo will take a long time or not work, because the target universe of "//..." is far too large. This is where other systems come in.

[0] https://docs.bazel.build/versions/master/query.html#rdeps