Looks like the Tanita devices are bioelectrical impedance. However, it appears it’s a step up as the measurement is taken through the entire body. With just scale sensors it will just assess your lower body as the current will just go up one leg then down the other. The ones I’m seeing also incorporate two sensors that you hold to get upper body as well.
Here’s what I found from their site:
All Tanita body composition monitors use advanced Bioelectric Impedance Analysis (BIA) technology. When you stand on a Tanita monitor, a very low, safe electrical signal is sent from four metal electrodes through your feet to your legs and abdomen. In segmental models, the four hand-held electrodes will provide extra readings for each leg, arm and abdominal area.
The electrical signal passes quickly through water that is present in hydrated muscle tissue but meets resistance when it hits fat tissue. This resistance, known as impedance, is measured and input into scientifically validated Tanita equations to calculate body composition measurements. Depending on the monitor, body composition measurements are provided in under 20 seconds.”