For reference, in a session last night the Warlock of the group had a summoned Beholderkin from the Summon Aberration Spell. A Lair action occurred which could have caused the Beholderkin to have been rendered Prone. I as the DM know that generally Beholderkin are immune to the Prone status, however it does not specifically state in the Summon Aberration spell that you use the stat block in the source book (i.e. Monster Manual) as it provides it own.
For reference the spell states:
You call forth an aberrant spirit. It manifests in an unoccupied space that you can see within range. This corporeal form uses the Aberrant Spirit stat block. When you cast the spell, choose Beholderkin, Slaad, or Star Spawn. The creature resembles an aberration of that kind, which determines certain traits in its stat block. The creature disappears when it drops to 0 hit points or when the spell ends.
The spell also provides a stat block to use.
So, which traits are used from the Monster Manual (or other source book) stat block in conjunction with the Summon Aberration stat block?