This is for the small turbo Escort Cosworth, I expect the large turbo is a little simpler.

The carbon canister acts a reservoir for the fuel vapours that are only allowed into the inlet manifold if certain conditions are met. The process for deciding purge solenoid dutycycle is;

The ECT has to be below 123.33 deg C.
Certain 'ECT at start' vs ECT and 'time since up to temp' vs 'time since start' criteria have to be met
Decide if purge at closed throttle is allowed (it is in the stock ecu)
The ecu has to be in closed loop operation (it can be calibrated to also do the purge in open loop)
Calculate purge solenoid dutycycle based upon airmass entering the engine (this varies from 4.69% at 384 kg/hr to 100% at 691 kg/hr)
Alter the purge dutycycle according to two purge timer values
Average the new dutycycle with the old dutycycle to prevent too large a change
Alter the purge dutycycle according to adaptive learning stored values.

As can be seen, quite a few decisions are made to calculate how much fuel vapour can be added without detrimental effect to the running of the engine.

Oh yeah, the valve near the filler is just a one way vent to allow air into the tank to compensate for the fuel used.