...
The most widely used systems by our customers are:
SAP 4 Hana: https://searchsap.techtarget.com/definition/SAP-S-4HANA
Salesforce: https://www.salesforce.com/products/what-is-salesforce/
Microsoft Dynamic: https://dynamics.microsoft.com/en-us/
IBM AS 400.
Some customers have their own built system.
...
For communication within the data integration process with these systems, CSV/XML files are transferred via one of these methods; SFTP or , S3 storage or API which these systems provide which are usually , or a REST or SOAP API call .It is good if the integration engineer has user or technical experience with some of these systems.(which is provided by these CRM systems noted above)