Sentinel-1 does not store soil moisture as a separate information. Moisture does affect the backscatter intensity of SAR signals, but so does surface roughness and vegetation cover, materials ect…
So there is no direct relationship between S1 backscatter and soil moisture. There are global missions (SMOS) which deliver soil moisture, but at a coarser scale.
Maybe these materials help you for a start:
- https://eo-college.org/resource/soil_moisture/
- https://eo-college.org/resource/soil_moisture_tutorial/
There are approaches to estimate soil moisture from radar data but they require field data, multiple images, or advanced workflows. They are discussed in many topics already, so I suggest to go through these topics and continue the discussions there: