The reason we talk about Robotic Process Automation is because these digital robots mimick human behaviour in many ways. Instead of a software that has to be integrated in the backend, RPA Robots log into applications with their own login-names and passwords.
It doesn't matter what kind of applications a company useses, whether they are self-developed systems or comprehensive ERP systems. Because the robot works with them like a human, it can use all of them.
Data has to be structured
One caveat is that, unlike humans, RPA's digital robots cannot handle unstructured data well. Robotic Process Automation is not artificial intelligence. They work on the basis of a set process workflow. Therefore, they only have as much intelligence and decision making abilities as the rules set down in this workflow allow.
As long as data is digital and structured, the robots can use it and work with it. This is why they may have problems understanding an email's text, but are capable of extracting information like the sender or the customer ID from that e-mail.
How the Robots use the User Interface
Lika a human user, robots can execute certain tasks. They can open programs or website, log in, click on links or buttons and fill out forms. They can copy & paste existing data or compare it with other data sources. They can extract data, update it and cross-reference it. The robots can also make decisions on the basis of predetermined business rules.
There is a reason for the robotic in RPA. They act a lot like humans, the biggest difference being a human's ability to handle unstructured data and the robot's ability to always follow a process exactly and error-free, no matter how often it performs the task.