1. I am all ears.
(What is the part of speech of 'all'? Is 'all' an adjective or an adverb? What is the meaning of 'all' here?)
2. They are all happy.
(What is the part of speech of 'all'? Is 'all' an adjective or an adverb? What is the meaning of 'all' here?)
"All ears" is an idiom or slang expression, meaning you're listening carefully. "All" is used as an adjective.
In the second sentence, all is a pronoun. It means everyone.