Knowledge Transfer from Software Development Company

Hi, I am a first time founder and I am not from technology background yet I am building a tech product.

I have hired a Software Development company for the same. While the development is still going on, I was suggested to start the knowledge transfer process now.

Can someone let me know how to effectively do that? What I should be looking out for and when is the ideal time to do that?

Naturally I will be hiring someone to do this only so if someone can let me know if there are certain skills I should be looking for in the IT professional for this job, it would be a big help!