Logistics of Find Familiar/Find Steed

[Greywander]

Do they need to eat? Sleep? Do you need to care for them, or do they just sort of exist immutably, ready to spring to your aid as fresh as the day they were summoned?

This is probably more relevant to Find Steed than to Find Familiar, as the familiar can be dismissed to a pocket dimension and resummoned at will, while a steed can only be dismissed permanently, requiring another spellcasting to resummon it.

[Asmotherion]

Well, they're technically spirits (Celestial, Fey or Fiend). I'd say they don't need to eat or sleep, they just can choose to do so.

On the other hand, they still need some kind of rest, it just does not involve sleep. Just light activity I suppose. And since I can't really picture your Steed or Familiar reading, it might as well pretend it's sleeping, or just stand there watching, I don't know.

[Millstone85]

If they do not need to eat or sleep, what about breathing?

[Scarytincan]

The summons assume the form of and have the statistics of the creature they take. The only stated exceptions are their types, telepathic bond, etc so I would rule that they need to eat and all that as normal since there is nothing in the spell descriptions that says otherwise. The descriptions state the only exceptional rules, with no further rules anywhere that say anything about celestials/fey/fiends not needing to eat that I am aware of.