I have a Project Type and my problem is, I want to create another DocType when I save my Project.
So when my Project type is Engine Repair then on Save I need to automatically create an Engine Repair Sheet connected to my new Project.
I would propose you set up a custom script on the project DocType which checks the project_type field, and on change creates a related engine repair sheet (maybe combine with set only once property of the field to prevent multiple creation). Could be something like
Conditionally change the dashboard entries is probably more difficult than useful. Either make a branch and change the dashboard, or maybe add a button “Open related repair sheet”…
probably not possible. Normally, you would create a new app and then install it, but you cannot do this of the ERPNext.com hosting as far as I know. If you need custom server side code, maybe you need to check other hosting options (that is the beauty of open source software).