Two questions spring to mind that are not immediately obvious from the website - how do they get access to your messages in different accounts?
Are they using some API? Or do they ask you for your username/password for each service and "helpfully" login for you?
They pretty much wrap the website versions of the services.
It appears to be just a wrapper around web interfaces. Electron or some such nonsense. Truckloads of megabytes - binary size as well as memory consumption.