You do not chase down, the buld system detects all affected modules and runs their tests. That's the advantage of monorepo - contineous integration that includes all dependent modules.
It's also a disadvantage, to be clear. Tests take longer to run when you need to rebuild your database and not just your own code. There's no easy way to put something in maintenance mode and only take changes for bug fixes, because maintaining forks is not a significant thing. Thus downstream dependencies must pay not just for bug fixes but also for feature improvements, deprecations etc.
It works well enough if everything is making money and is being actively developed.
