How is QB64 written?

[TDarcos]

So, would it be helpful to learn C in order to better understand and/or contribute to QB64PE?

Not necessarily, unless you want QB64pe to have some other feature that is not capable of being handled by existing code and which you're unable to sweet talk, err I mean cajole, someone currently working on the program to put it in. If you want to get better as a programmer, read programs. Lots of them. I have, and I still do; there is no such thing as learning "too much." And I've been writing programs since I first learned Basic, oh, over 45 years ago. You could take a look at QB64pe.bas; it's located in the source folder in the folder where QB64pe is installed. But be advised, compilers can be very complex. Or look at some archives of Quick Basic software and examine it. My own collection of Basic language source code, mostly in zip files, is about 8.5 GB, over 60,000 files. I have a lot more reading to do...