Agree with DrByte that PHPStorm is worth trying/using. I have/use it when I need to do some more "intricate" review, coordination, evaluation though more frequently I enter code directly I'll say into the file. Like anything there is a learning curve and to say the least it is very powerful. Back in the day I had used some other IDEs the names of which I forget, but its great to have the additional tools of code completion, assignment/use lookup, it even has a git interface though I have a separate tool that I have come to like because with that tool I can also visually look through the history of the commits, see my branches, do branch/commit comparison, etc...

As to the question about the order creation. Again, it is good to indicate what is it that you are trying to understand or question to answer? Some questions that come to mind is, do you just want to see what data needs to exist prior to initiating creation, do you want to see how the order gets created, how its used after it was created, everything?

Because no pre-knowledge is identified, what I would suggest is to start using and becoming familiar with the Developers Tool Kit found under the admin's tools menu. The broadest search capability is found in the lower row of that tool. For order related information, I would suggest to start searching on "new order" (without the quotes) May even want to narrow it down to: "= new order" such that text with just the first phrase doesn't appear in the search. Then I suggest also changing the number of lines above/below to at least a value of 3 instead of the default of 0. (BTW, that is a default in the file admin/developers_tool_kit.php that can be changed at/around line 10 of the file.)

Once you find instances of that text, you can 1) follow the logic of the class' constructor as to which way it goes and what is available, 2) work backwards to see what was done to get to that point and how other classes and code work together to get there.

And lest didn't know/realize, help spills out when it is seen what has been tried, what is working, what is wanted, what doesn't work, etc... One thing seen (and I've said something similar before), those that show that they are trying to help themselves get help even with things that are difficult to accomplish.