The Si1145 is a sensor with a calibrated UV sensing element that can calculate the UV Index. It can communicate via I2C communication (address 0x60). You can hook up this sensor with I2C port in the PCB which is located at the bottom left of the ESP board.

The SI1145 sensor really doesn’t have an actual UV sensor! Instead, it looks at the amount of visible and IR light it receives from the Sun and uses a formula to calculate the UV index, right down to two decimal points.

You can read this article to know more about the UV sensor.

If you need a more accurate measurement of UV Index, you may use the VEML6070 sensor. Unlike the Si1145, this sensor will not give you UV Index readings. However, the Si1145 does UV Index approximations based on light level, not true UV sensing. The VEML6070 in contrast does have a real light sensor in the UV spectrum.

You can read this tutorial for writing the code and using its library.