The main activity - the company is engaged in the settlement of damages after an incident with the client's vehicle fleet. They work only B2B with fleets of 80 cars or more. They act as a third party that takes over all the tasks instead of the client.
These 11 companies did not develop at the same time in terms of IT. Therefore, these are about 7-8 systems integrated with each other to some extent, but not to some extent. Since 1.5 years ago they were still independent companies. Now the IT department has a big task - to adjust the IT landscape by changing the business processes themselves and, within this framework, the integration of multiple systems, their development, and support. There are purchased systems, and there are self-written ones.
Java, starting from Java 6 to 11, Sim with certain restrictions, it is already deprecated and requires a certain version of highborn, and if you remove the sim it is impossible to even upgrade Java 6 higher.
The new part - there is Docker Swarm, (its own data center, not cloud) a set of services that are spinning, these are not microservices, but according to their gradation, these are correctly separated services. There are already Java 11, unit tests, integration tests.
In the old system, there are no tests, no documentation.
There are people who can tell you something. The global idea is to refactor this old system and translate it into a new one. This is not refactoring for the sake of technical refactoring, here both those and ideas are rushed. The challenge is to review the components of the system and examine whether the functionality needs to be changed. If necessary, then perhaps it will not be refactoring, but the development of a new component that will replace the old one.