It probably wasn't outsourced. Local authorities have their own GIS and data people because it's essential to a council being able to do most of the jobs it does.
Keeping track of things is a really necessary function of any council. They have massive asset registers that need to know where everything is, from streetlights, to bins, to bus-stops, not to mention planning. They track anything they are responsible for, because otherwise it's rather hard to be be responsible for it.
That includes street trees, which are the council's responsibility. This is why the London one doesn't include ones in parks, since those are maintained by the Royal Parks people. This is their internal register of trees that someone has slapped a
My local council (Bath and North East Somerset) has a small but excellent data team, and they have an online map with all their data in one place. There are layers for everything, https://www.bathnes.gov.uk/webforms/maps/