Basically just to confirm, the final two are employed in temporary posts during the gap between filming the series and broadcast. The final task and boardroom is ultimately pointless - the winner is decided on the time they spend in the company, though of course, performance on the show will be taken into consideration.
Sugar doesn't tell anyone who the winner is until a few days before broadcast of the final - even though the show filmed a year ago, Stella presumably only found out she won very recently. During filming, they record two seperate sequences of the winner being announced, probably explaining the completely underwhelming victory moments.
Stella now starts her actual role in the company now that the show has been broadcast, and it's public knowledge she's the winner.
* All this coming from what past finalists have said.
Of course, the strange bit is, you would think there would be final two spoilers out there every year, but as far as I'm aware, that has never been the case. There was one this year that said Chris and Joanna were the final two, but that could have been guesswork.