How is analog current converted to USB?
An analog-to-digital converter (ADC) will convert analog signals into a quantized digital value. I can't say I have ever used the symbol in a schematic, but Wikipedia says to looks like this:



An ADC isn't the whole story with getting analog to USB though. You'd also need a device to collect the digital data and package it for transmission over the USB. That could be a few different things. I would use a microcontroller. The schematic symbol for that varies greatly, it's a descriptive symbol rather than a representative one, if that makes sense.

In your case the ADC and controller and inside the sensor device. If I were just doing a representation of it, I would show the sensor/adc/controller as a single abstracted component, which outputs both digital and analog signals. You may require more detail.