First you need to synchronize the oscillators of two SDR receivers. The system clock isn't nearly precise enough.

So you just count the samples you get from the SDR, that turns into a timestamp that just starts at 0 when you start dump1090. The mlat-client gets that timestamp with every message it gets from dump1090. Certain messages are sent to the server for synchronization.

For sync ADS-B location message pairs are used. For an ADS-B location message you don't know when it happened, but it is sent at a certain point in time. Using its location you can calculate travel time to the receiver who received that message.

Now if two receivers get the same message pair and that information makes it to the server, the server can build a conversion how to translate the oscillator timestamps of the one receiver into the clock of the other one .... then you have a synchronization.

Now you need at least 4 synced receivers .... and then you can calculate a position from the arrival time at the 4 receivers for any message you choose. suffice it to say ... it's somewhat complicated.