I've started documenting our implmentation with Concur Invoice. It might be helpful for you to follow - or - not. We are interfacing to SAP. I'm not sure if it makes it harder or easier.
The first of probably many blog posts:
https://blogs.sap.com/2020/05/20/sap-concur-blog-1-interfacing-is-interesting/
I'm tending to double up my questions here and on the SAP Community. There is a space for us there. (If you are integrating with SAP) I'm told the cloud version is seamless. We don't have SAP on-the-cloud. We have on-premise. So there is a lot of work to be done.
Here's the tag if you'd like to try for an answer in there as well.
SAP ERP integration with SAP Concur solutions
