Wait a minute, so you didn’t know this all along! Goodness gracious! You are indeed clueless. Anyway, its good you clarified that for us.
The nested hierarchy is basically all you need to know.
The logic is really simple:
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Common or vertical descent produces a nested hierarchy.
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If all cellular (unicellular and multicellular) lifeforms fit into a nested hierarchy (or groups within groups), then it means there was universal common descent.
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All lifeforms do indeed fit into a nested hierarchy and so we can conclude they all shared a common ancestor.
~3.5bya, that universal common ancestor was a population of “cells”. LUCA had contemporaries too and they might have contributed to her gene pool, but she was the one from whom we directly descended from.
The question is difficult to parse. State it clearly.