The affected creatures retain their other types and abilities.

Creatures that die while you control Ygra are still Food artifacts as they leave the battlefield, so Ygra's last ability will trigger whenever a creature affected by its second ability dies.

Replacement effects that modify artifacts as they enter will apply after you apply Ygra's effect. For example, if you control Ygra and there is an effect that says artifacts enter tapped, creatures will enter tapped.

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).

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.

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