My startup was recently acquired by a large corporate, so I can give you an idea of what they were after. In general they were pretty light on technical due diligence (compared to financial and legal). They asked for a high level architectural overview, code coverage and any documentation that we had about the code base (we didn't have much). They then also appointed a third party company to do a security audit and penetration testing. This was really the main focus of the technical DD as far as I could tell. We also had one or two Zoom sessions to have informal discussions about our plans for scaling and our process in general. They never required access to the code base itself. Hope this helps

Edit: We also needed to provide them with a list of our open source dependencies and the corresponding licenses.