It is not possible to build systems in isolation that don't interact with one another. Even though we are building separate services to address the requirements of different business domains, we eventually need to make them interact as a whole in order to meet the demands of the business. This interaction is carried out by using application programming interfaces (API).
"An API is a set of commands, functions, protocols, and objects that programmers can use to create software or interact with an external system. It provides developers with standard commands for performing common operations so they do not have to write the code from scratch."
- API definition from https://techterms.com/definition/api
Monolithic applications tend to carry out database integration. This is something that should be avoided at all costs; any required interaction between services should only be done using the provided service APIs.