The description of the find familiar spell states:
Finally, when you cast a spell with a range of touch, your familiar can deliver the spell as if it had cast the spell. Your familiar must be within 100 feet of you, and it must use its reaction to deliver the spell when you cast it. If the spell requires an attack roll, you use your attack modifier for the roll.
Does this mean that since the familiar casts the spell, it can also concentrate on it?
Best Answer
It is you casting the spell, not the familiar
The initial assumption "the familiar casts the spell" is incorrect. The spell description explicitly says you cast the spell, not the familiar:
The familiar only delivers the spell, not casts it. At the moment it delivers the spell, the spell was already cast. The familiar does not concentrate on it, neither it performs any spell components. It only makes a spell melee attack, albeit using your spell attack modifier.