> How does that work?

My gut feeling is that any serious real-world company with a proprietary codebase worth looking at would not be handing out the crown jewels to a third party. License or not.

I don't doubt somebody licensed their codebase to them, I just have my doubts about who the "who" could be.

lot of ads everywhere offering to buy your codebase of real product/star up even it long gone or failed (offer usually price per lines of code). So most likely that they have bunch of abandoned codebases between small and medium sizes and probably also some fake codebases as well.

From the same site: https://withspecific.com/company-data

we reached out to companies that were willing to license their codebases. every codebase we used had real users, one of them had 200k+ users and is currently top 100 on the app store.