The platform gets voted on at conference by delegates. The majority of delegates at this year's conference are likely to be quite similar in make up to those at last year's, so don't expect too many changes to official policy (expect Tridant renewal to remain, for example).
There are 2 wings to the party, in practice (discounting the members, as they are wont to do lol). The leader, cabinet, MPs, etc, and then the administrative wing, which is McNichol and the NEC. McNichol has a lot of power. It's not an elected position - he's a civil servant, employed by the party, and has a contract like any other job (which is why he can't simply be gotten rid of). The NEC is generally the main administrative body, and in recent times McNichol and the NEC have been in step. McNichol is officially accountable to the NEC, but there are practical limits on that accountability. Now the balance of power has shifted somewhat in the NEC, McNichol's job might be slightly more complicated. The NEC vote on any administrative rule changes to the party. The exception might be if it's a significant change - like one member one vote, or changing the role of the Chair - in which case it would likely have to go to conference as well.
In reality, the leader can end up with little actual power, but nevertheless is generally deferred to (as long as he's not too left wing
). If McNichol and the NEC don't like the leader, as we have seen they can run the party how they want instead.
And then of course we have all the officials at regional level. They're also unelected employees, usually put in place by the NEC (to my knowledge). Many of the 'reluctant Corbynistas' who are long time members have been pushed over to Jez's corner in part because of the years-old culture of stitch-ups and bullshit from region. The way region is run makes a mockery of any pretense of 'democracy'. NEC and region are how and why there are parachutes that local members can usually do precious little about.