Leasehold is a huge subject.
there isn't really a set "how it works" - it will depend on your lease.
The only things, I think, that are set in stone, will be:
you pay ground rent
Many blocks have service charges for regular cleaning and maintenance, gardening if there is one. Mine includes my water as well.
Some blocks run a fund and manage it themselves with leaseholders involved in the management and allocation of money, others don't.
I'm currently in one that doesn't have any leaseholder involvement and I regret that now as the freeholder is not incentivised to get value for money for our service charge and we don't have a way of sorting that out.
So if you can get one where leaseholders get a say in how money is spent, I would do that. I realise it's not always possible with flats though.
Having a full service managing agent does mean not having much to sort out myself though. You can get leaseholder blocks where the leaseholders themselves do the management as well. So there's a big variety.
these things might be useful
www.lease-advice.org/publications/documents/document.asp?item=7