How can I put my own road network instead of this default road network
You need an ArcGIS Enterprise for that.
If you have ArcGIS Enterprise, then you could Publish routing services—ArcGIS Server | Documentation for ArcGIS Enterprise.
The REST URL of the routing service, can then be used in WAB.
Jayanta is right. I have also included some really useful resource on how to build your road network which you might find helpful 🙂
https://www.youtube.com/watch?v=odzI3rC5ErQ&ab_channel=EsriEvents
https://pro.arcgis.com/en/pro-app/latest/help/analysis/networks/network-analyst-tutorials.htm