Electronic – what is the meaning of the pipe symbol “|” in front of a variable

You should completely ignore the terms "behavioral", "Register Transfer", and "Gate level". I think these terms were invented by professors or people writing text books and have little or no relevance to "actually getting work done". It's like scientists and professors inventing Latin names just for the sake of making them sound smart.

Write your code (Verilog, VHDL, or whatever) so that you get the results you want and your code is readable and understandable. After you have done this a while you will settle into a style that works for you.

I should mention that the only way this can happen is if you learn everything about Verilog (or whatever). Often there are features of the language that are not covered very well by books or professors but really help you efficiently write code that is easy to read.

Synthesis is highly dependent on the platform you're using and usually needs to be done by tools created by Altera, Xilinx, etc. Nothing open source exists (AFAIK) because this is so custom and requires a lot of effort to obtain optimal and correct results. Therefore, there's little incentive to do open source. Also, because of the IP, these companies don't share information about their chip internals, which prevents others from using them without going through the manufacturers.

By the way, Altera and Xilinx (perhaps others) provide free versions of their tools with some features missing you can use (which is another reason no one seems to do anything open source). They're good enough for many projects.

So, to summarize, do you think anyone would spend time, for no money, to create something that is difficult, with little information, when the manufacturer already provides some of it for free? Take a look at the open source BIOS for PCs. Hasn't gone very far for these same reasons.