Starting with Syntex - Hierachy Question

Document understanding models that you create in the content center do not need to be applied to the document library in the content center but are applied (as you mentioned above) to other document libraries. Let me see if I can summarize:
You create the model for "Contracts" in the content center. Once your model is tested and ready to publish, you publish the model to the Finance Document Library for your finance user. Document Understanding models are deployed to a document library (not a folder although that may be a nice feature request for the future). The finance user will upload content to the folder and those documents will be evaluated by Syntex and the Contracts model applied if it matches the classification tests for the model and then the metadata extracted.

If you have more than one model applied to a library, all of the models are evaluated in parallel and if multiple models match the content the one with the highest classification score will be used.

If you want to apply a model to multiple document libraries, that is also possible for Document Understanding Models. Just publish the model to multiple libraries and you are good to go.

For your second question about a "listing or view", you can always use the contentype to return the content that matches (e.g., one view for Purchase that returns PurchseContracts and another for Finance that returns FinanceContracts) if I understand your meaning. These are not permissioned views though but more of a filter. If you want to have permissions, have one folder for each ContentType and either have people upload to the right folder or us a Flow to move the document to the right folder with metadata after it is classified and extracted by Syntex.

Hope this helps.