What is collective ijtihad? I would like a detailed explanation of this topic.

Collective ijtihad is when a group of the Ummah's scholars — often with specialists — convene to deliberate on the ruling of a certain issue. This type of ijtihad is not new; Abu Bakr and Umar, may Allah be pleased with them, practiced it. It is more conducive to reaching the correct conclusion due to the cross-pollination of minds and the way they complement each other, even if some participants have not reached the rank of individual ijtihad.