Indeed, you have no access to the source code and thus can not edit the application. It is licensed as proprietary software (see for example What is proprietary software and how does it work? on what this entails).
If you download a Desktop version the code-base is similar to what the Cloud and Server editions have except that the latter two have additional code for multi-user use and authorizations. You can not edit the Desktop nor any other edition as the code is kept secret by the owner.
Remember, @Giovanni, what you buy is a license to use the software (with updates for one year). You will not own the software. This is like every other piece of commercial software in the world that I know of.
As far as I am aware there is not bespoke support. All Manager instances are identical. You can customize Reports and still (although labelled obsolete) Themes.
You can post your ideas in this forum and if it gets traction it may be promoted by a moderator or the developer to the ideas category. Then depending on many factors it may be implemented within an unspecified time period. If you check the ideas category you may notice that there are quite some old and new ones.
That happens all the time. Manager is a continuously evolving product. However users can suggest but not dictate what changes are actually implemented.
Despite this, much can also be achieved by customisations outside of the NG Software code base.
@lubos We are testing the tool and we would need some changes in some calculations.
In case we need any changes, can you provide us help? and how does it work?