I'm curious why T2 is needed. Couldn't T1 be wired to direct 12v to the relay alone just like the role of T2? They're both the same part numbers
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Best Answer
That arrangement is known as a Darlington pair. It gives a current gain of the product of the gains of the individual transistors.
simulate this circuit – Schematic created using CircuitLab
Figure 1. A more common Darlington arrangement.
A small current injected into the base of Q1 will cause a large emitter current which will flow into the base of Q2 causing an even larger collector-emitter current in it.
Theoretically, if Q1 has a current gain of 50 and Q2 is the same type then the combined pair have a current gain of \$ 50^2 = 2500 \$. This allows tiny signal currents to switch large loads.
In your schematic the small signal is provided by the PIR sensor. The relay forms the "large" load.