Programmer Types:
Do you write extensive design documents for your games?
Is using UML the standard method of diagramming your use cases?
Do you run the whole gamut of diagrams or sort of pick and choose along the way depending on the scope of your software and the number of developers on the team?
And regarding designing for Unity, do you find it difficult to merge the workings of Unity into those diagrams?
No
No
No
And… No
That was easy.
Programming and design of that programming is largely based on how loose or tight you want to control your program. Also, it largely depends on how many people are involved in the process. If you are playing all by yourself you probably will never touch UML or build diagrams based on it, but if you are running 10 or more programmers it is probably a complete necessity. My answers are all no because I work alone. I like it that way, no one tells me what to do except the little voices in my head. 
I’ve been in the game industry for about 15 years. I have worked on the side by myself and I have worked on a team with 50 programmers. All I know about UML is what I learned in a class I took once. I have never used it and never felt inclined to use it.
With that said however, if you feel it helps you, then by all means use it. It is a tool; use it if it helps you.
In a team with 50 programmers, how do you know what each of you is assigned to? How do you know how the different modules will connect to each other? I’m inclined to think that without any kind of documentation that working on a single application with 50 programmers would be very chaotic, and inefficient.
Yes, with a large team, there will always be chaos and inefficiency. Many would argue that once a team gets too big that adding another programmer actually makes the project take longer. The best way to counter this is to divide the project into sub-teams and have lots of communication between sub-team leaders. It was very rare that we ever had a meeting that included the entire team.
To answer your specific “how do you know…” question, you don’t. The guys working on the front end had no idea how the AI works, for instance. People only know about the systems that affect what they are working on. We got the job done, but it wasn’t the type of team that I enjoy working on, and I don’t work there anymore.