Here are the steps implemended in the flow
1. Create a string variable that contains the csv output and initialize the header information
2. Query items from the first data source (in our case a SharePoint list) using the SharePoint Get Items flow action
3. To insert data in the csv string variable in the predefined column order use the Data Operations – Select action
4. Convert the output from the selected action into a csv file. After that extract the lines ignoring the header info and ignoring the empty line at the end of the csv output.
5. repeat steps 2 till 4 to enrich the content of the csv output string variable with content from other data sources
6. replace the placeholders in the csv output string with the right line delimiters used in PowerApps and return the output string