When It Is Used As An Ammeter What Is The Equivalent Circuit When Used As An Ammeter ? How Does It Effect The Circuit Where It Is Used For Measurement?

Some ammeters have a shunt resistor, and the effect of inserting an ammeter into a circuit will be the same as inserting a small resistor into the circuit. There will be some loss of voltage across the ammeter.  Other clamp-on ammeters used in AC circuits will act like a small transformer inserted into the wire (mostly mutual inductance plus some leakage inductance).

Any measurement device requires some draw of energy.  Modern instruments strive to minimize the loading effect of the instrument, but there is always some effect.