Dear Anonymous A,
I shall speak from my McKinsey experience.
At McKinsey, Associates rarely conduct First Round interviews. It's probably more accurate to say they never do, but I cannot be certain.
Typically, Associates who have successfully transitioned to the Engagement Management role and continue to be rated highly (the top tier that includes the roughly top 20% of performers within the same hierarchy) are tapped to contribute to the firm's talent acquisition by becoming interviewers.
Once an EM is tapped (and ultimately accepts to interview…I never met a single EM who said No…McKinsey is not a place where people generally say No in such circumstances), they go through a training course, are given access to the rotation of interview Cases for that office, and then assigned to support Recruiting.
I have heard of smaller offices where EMs outside the top-performing tier were also tapped, but this likely would have been due to a sponsoring partner with clout vouching for the EM's ability to serve office and firm well in this regard.
I can't see analysts (senior or not) being tapped to conduct interviews!