Well ...
A digital scale app on Android usually uses the accelerometer to measure how far and how fast the phone was "pushed", which can be converted to weight. Very roughly. You have to place the item on the phone very gently, or it's going to measure the effect of the item falling too. It's certainly not going to be accurate, no matter how it's written, and no matter how you place the item on the phone.
There are a few "real" ones in the Play store, but the best of them is really a gag app, since the accuracy depends on you and can vary by amounts like +300%/-80%.