2 Questions regarding XTR105
The circled op-amp controls the loop current by controlling the base of the transistor Q1 or 'component 2'. 

For example, if Vin = 0, the 100uA current source causes a 97.5mV voltage across the 975\$\Omega\$ resistor, which causes a 4mA output current ( 1 + 975/25 = 40). Similarly, if Vin > 0, then Io = 4mA + Vin(40/Rg). 

The circled transistor does the same thing as Q1- controls the output current. If Q1 is present then it carries little current and Q1 takes over. If Q1 is absent the circuit will still function. 

However, in loop-powered instrumentation the loop voltage can be quite high, so the power dissipation at full scale (30mA is possible) can also be quite high, so it's frequently desirable to offload that power dissipation into an external component that can handle the dissipation rather than have it on the same die as sensitive circuitry and may not be capable of the dissipation).