With SP 2019, is it possible to integrate MS flow and Power Apps?
Below is my assumption, please suggest me if it is not true.
SP2019 is onpremises server. To work with MS flow and Power Apps, users need to buy licenses separately as described here for MS flow pricing, PowerApps pricing. Install on-premises gateways and connect to MS flow and PowerApps cloud then implement the solutions.
Every user should have appropriate license to use them or view them.
Alternatively, you can make use of ready-to-use 3rd party tools. Basically a Windows Service running locally to forward changes in SharePoint (or almost any other app) to Flow can do the job. No user-based licensing required in this case - just one-time per installation. Hope that helps, Frank.