Each time a card leaves your graveyard in a turn, it counts as one of three cards the activated ability is looking for. For example, if you return two cards from your graveyard to your hand, discard one of them, then exile that card from your graveyard, you can activate Bonecache Overseer's ability that turn.

If an effect refers to a Food, it means any Food artifact, not just a Food artifact token. For example, you can sacrifice Tough Cookie (an Artifact Creature — Food Golem) to activate Maraleaf Rider's ability (an ability with "Sacrifice a Food" in its cost).

Food is an artifact type. Even though it appears on some creatures, it's never a creature type.

Whatever you do, don't eat the delicious cards.

You can't sacrifice a Food to pay multiple costs. For example, you can't sacrifice a Food token to activate its own ability and also to activate Maraleaf Rider's ability.