A small vocabulary point
As its name suggests, a web documentary is an interactive documentary to be viewed on the internet. Very popular in the early 2010s, the goal is to immerse the reader in the environment covered by the web documentary. One of the most successful examples is probably Fort McMoney by David Dufresnes.
A long read is a lighter format that is very popular with the mainstream media when they want to highlight a story. Long read articles take a fairly traditional form, but they are usually enhanced with background images, animations on maps, embedded videos, etc. The work carried out by Longreads for several years now is a very good example of the long read format.
Working as a team
Few journalists are capable of producing an interactive long format alone. These web items generally require additional skills to those of a journalist. They require a developer to code the item on a webpage, a graphic designer to ensure a certain aesthetic cohesion, sometimes a video maker and sometimes a composer.
In short, embarking on an interactive long format is often an opportunity for the journalist to polish other skills. When embarking on a project of this kind, it is advisable to have strong allies and, in particular, not to expect to finish it too quickly.
A fairly fragile economy
The problem is that media outlets are generally unable to allocate the resources required for projects of this kind on a long-term basis. Despite many attempts, few interactive projects have really found their audience or made their development profitable.