From a local C# program, I am trying to connect to an SSMS server's Integration Services Catalog (SSISDB) to read the .dtsx file(s) that are in there. I have the IP/u/p of the server, but after that, how would I get the .dtsx files?
You can extract SSIS projects (including dtsx files) using the SSISDB.catalog.get_project stored procedure. You should use it to save the project as a ".zip" file and extract the packages from it.
You should call this stored procedure from an SQL Command using C# to extract those files. While searching online I found a solution implemented in PowerShell, you can implement the same logic using c#: