If you want to measure current directly, the ammeter needs to be fitted into the circuit ...
The current now flows through the ammeter. This is a direct consequence of the Kirchhoff current law (KCL). The value of current in ampere [A] you read now directly from the ammeter.
But pay attention to two points...
An ideal ammeter has no internal resistance (Ri = 0 Ω).
The current can flow unhindered through the meter and therefore has no influence on the measurement result.
An ideal ammeter is unrealistic. But what is a real ammeter and what influence does it have...?