How to organize automated testing of microservices?

Question

Automation

I'm a QA, I worked with Selenium + Java + TestNG, now I've moved to a new company and I'm stuck because I can't figure it out on my own. Please help with advice and direction.
Given: We have a project, several microservices. There is no front end. We manually test approximately the next. Scenario: we generate a csv file with data, in the form of tables, for example, full name, employee number, date of birth, etc., let's say we changed the date of birth and sent it through win CSP, and through the microservice to the database. When the microservice processes everything, it sends us an xml output file. In it, we check whether everything meets expectations.
Question: how to automate business logic from scratch?

We use: typescript, jest,
The application interacts with Kafka, mongodb, SQL DB.