You still have to understand your boundaries when you have a large monolith unless you have one big ball of mud.

Even with a well constructed monolith, you need to have well defined “services” with contractual interfaces.

You don’t have to understand 4000 services to make one change anymore than I need to understand the entire boto3 library when I am building on top of it.

https://boto3.amazonaws.com/v1/documentation/api/latest/inde...

You can’t just change your interface in a monolith either without breaking other parts of it.