I did happen to see them at work before...
So there are 2 ways how they get them delivered.
1) Part-timers or couriers pick up at warehouse.
This is the most common. These pickup points tend to be at warehouses one. They just go there, pick or get allocated which parcels to deliver. Items get scanned and they go and deliver.
Some workers will bring those big Ikea blue bag and stuff as many parcels as they can to deliver. While some just put it in their car/van then go and deliver.
These get paid like $1.50-2 per packaged delivered.
2) Sub-distribution points
So here's what happens: all these packages get picked up from pickup points and whatnot, and sent to their warehouse for sorting. They sort by location and area... So, for example, distribution point A, B, C, .etc.
Once it is settled, a huge lorry/truck with these parcels will go to these distribution points.
The lorry will then park at those HDB estate areas. Those part-timers will come and collect the parcels from this lorry to deliver them to those homes in the HDB estate areas.
Sadly, they get paid the least but convenient since it is nearby and no need to go all the way to the warehouse to collect.
They get paid like $1 or less per package delivered.
That is why sometimes you never tell them to leave on doorstep but they still do it because they just want to mark the item as delivered and earn that $1-2. You can't expect them to wait at your doorstep, ring bell, then wait... Time is literally money so they just put on doorstep and leave liao.
So in regards to this thread, it likely falls on the 2nd category... They throw all the items at void deck for the part-timers or workers to go deliver. In the past, it used to be collect from lorry and deliver... But I think due to time constraints and to deliver more parcels, they decide just to dump parcels at void deck and staffs slowly go and pick to deliver... Sadly, no security and parcels will get stolen easily. So if you ever notice your parcel wasn't delivered despite it says 'delivery in progress' or 'at sorting facility'.... Well, now you know.