Installing a dimmer switch
You need to use a voltage tester while the circuit is on to identify which is the load and which is the line. The line comes from the circuit break to the switch. As long as you have power, that will be hot. The load is from the switch to the light/outlet/etc. That will only be hot when the switch is in the on state.
Neutral is its own wire. Generally white, and will be in the back of the junction box capped (unused). You connect the smart/dimmer white wire to this wire.