I couldn't find the precise reference that mentions that they found the source code for the Basic interpreter and just "copied/ported" it. I did read they'd go "dumpster diving" to learn assembly. But not that they found and just ported the source code. Where is it?

I think it comes from a misread of the text in the gates interview linked in the comment:

"r. We were moving ahead very rapidly: BASIC, FORTRAN, LISP, PDP-10 machine language, digging out the operating system listings from the trash and studying those. Really not just banging away to find bugs like monkeys[laughs], but actually studying the code to see what was wrong."

My understanding is that they saw the source implementation for other BASICs (on mainframes or whatever they were called at the time) but their code is mostly their own. Few if any programmers spring fully-formed from the head of zeus (although paul allen was close) and plenty of valuable intellectual property was originally created elsewhere.

"Just porting" is doing some seriously heavy lifting, if it's referring to porting something from a mainframe to one of the micros of the day.