Once someone signs their offer letter that's when we do the handoff from Recruiting to HR. After the offer letter is signed we start our onboarding process (which includes background check, training, paperwork, etc.). We use Greenhouse Onboarding which has made this process super smooth and informative (and fun!) for our new hires.

I typically see hand-offs to HR once the offer is accepted.  We're hearing more about counter-offers, the need to "rerecruit" through what's inevitably a tenuous time starting a new job.   aHAND-OFF might be more appropriate.  I've been fortunate to have recruitment or staffing coordination teams available to handle pre-employment paperwork and background checks.  Fortunate in having people to focus on this but it adds to the complexity of hand-offs.  Kudos to you for asking the question and being intentional about this critical inflection point.  The partnership between TA and HR is critical for the full-employment life-cycle and is worth cultivating.

We are currently in this phase at my employer since we are growing we have never had any HR staff members that specialize in recruiting.  Now we are to that point and we are working to figure out where that transition is and what recruiters are responsible for and what the HR business partner is responsible for.  What we have decided to do is have the recruiter post the job, review resumes, pre-screen, send out assessments, background drug screen, and potentially the offer letter.  Once they accept, the HR business partner takes it from there.