Note: When you connect the receiver to FC, please don't forget to plug the battery, otherwise the reciver will not work at all because if you don't plug the battery, the reciver doesn't have any power to work.
1)Here are the connection diagrams of Frsky R-XSR receiver with F405 Brushless FC.
Step 1 On Betaflight GUI, configure the UART1 port to telemetry output in Ports tab and choose smartport.
Step 2 In the Configuration tab, for Receiver Mode, choose Serial_based receiver (SPEKSAT, SBUS, SUMD); Serial Receiver Provider, choose SBUS, turn on TELEMETRY. And click the Save and Reboot button in the lower right corner.