A force sensor is a set made up of sensitive elements: strain gauges and a test body generally a metallic material, on which the strain gauges are glued.

Thus a force sensor can be assimilated to a transducer, that is to say that it allows via a linear relation to transform a mechanical physical quantity into an electrical physical quantity.
The strain gauge has a resistive impedance expressed in ohms (Ω).
Below, the relation between the relative variation of the resistance value of the gauge which is linked to the relative variation in length of the test body according to:
∆R / R = k.∆l / l

k is called the gauge factor, it is a constant.

The major technologies used for the production of strain gauges are: gauges with film screens on an insulating support, semiconductor gauges, thick-film gauges with screen printing inks and thin-film gauges by means of evaporation on a substrate.
The most used gauges and very far are the gauges with film screens made of Nickel-chromium, platinum-tungsten, constantan because one can modify via heat treatments the physical characteristics of the strain gauge for example its resistive coefficient in temperature ( CTR).

Why use a 4 strain gauge bridge to make a force transducer instead of a single strain gauge?
It is quite simply the precision obtained and the possibility of compensating for physical phenomena with the Wheatstone bridge.

Strain gauges have different profiles but the most commonly used have:
straight gauges and shear gauge, see profiles below: