First there’s a dedicated button to open the fuel door the also opens the fuel tank isolation valve & depressurized the tank then unlocks the filler flap.  I’ll let the owners manual take it from here:

FUEL FILLER FLAP LOCK
Note: Make sure that the vehicle is refuelled shortly after the refuelling button is pressed. Failure to do so may result in the fuel tank not being filled to the required level. If in doubt, check the fuel level gauge in the instrument panel before continuing the journey, to prevent running out of fuel.

Plug-in Hybrid Electric Vehicles (PHEV) have a locking fuel filler flap and an isolation valve on the fuel tank.
The vehicle cannot be refuelled until the following has occurred:
• The isolation valve on the fuel tank is open.
• The fuel tank is depressurised.
• The fuel filler flap is unlocked.
To enable refuelling, carry out the following:

1. Switch off the vehicle’s ignition, apply the Electric Parking Brake (EPB), and make sure that the transmission is engaged in Park (P).
2. Press the refuelling button located in the lower switch panel at the driver’s side of the fascia. The isolation valve on the fuel tank opens to depressurise the fuel tank. The fuel filler flap unlocks. The instrument panel displays a message to confirm that the fuel filler flap is unlocking.
3. The instrument panel displays a message to confirm that the fuel filler flap is unlocked and the vehicle can be refuelled. See 382, FUEL FILLER FLAP.

The isolation valve on the fuel tank remains open for approximately 10 minutes, at which point the valve shuts, preventing further refuelling. If further refuelling is required, repeat the above process to open the isolation valve on the fuel tank.

Note: Make sure that the vehicle is stationary. The fuel filler flap does not unlock, and the isolation valve on the fuel tank does not open, if the vehicle is moving. The instrument panel displays a message to confirm that the vehicle’s speed is too high.

Note: Make sure that the fuel filler flap is closed after refuelling. The instrument panel displays a message to confirm that the fuel filler flap is not fully closed. If a fault is detected on either the fuel filler flap system, or on the isolation valve system on the fuel tank, the instrument panel displays a message. In this event, consult a retailer/authorised repairer or roadside assistance. See 501, ROADSIDE ASSISTANCE. See 357, PROTECT.