Why is there an article "a" before "sea"? Is "sea of plastic" a countable noun?

Does the word “Sweetie” can only be used to little kids?

Yes, mostly, "sweetie" is used towards children. Ex: How was your day at school, sweetie?

When can I use the expression "someone bolted"?

"Bolted" can be used to describe a person or animal who suddenly starts running very fast, usually for a specific reason, like being late or frightened. Ex: She bolted when she found out she was late for work.

Is it "train of something" a common expression?

The phrase "train of thought" is actually a common expression but here it is also used as a literal name for the train. "Train of thought" is an expression that is used to refer to the process of someone's thinking or the series of ideas or thoughts that someone has. Ex: He interrupted my train of thought. Ex: I just had a weird train of thought.

What does "microcosm" mean?

A microcosm is a place, community, or situation that can be considered as representing in miniature the characteristic qualities or features of something much larger. For example, a town that reflects the characteristics or demographics of the whole country it's in. It would be considered a "little world" or "microcosm" of a larger whole. So Tim Cook is saying here that the ATT debate is actually representative of a larger debate that they had been having in the company. Ex: This snow globe is like a microcosm of my city during winter. Ex: We sampled subsets of the population to draw conclusions about the whole population. This is an example of a microcosm.

What does "benefit of something" mean? Is it general expression?

To "give someone the benefit of the doubt" is a common expression that means to trust or believe someone to be honest, until proven otherwise. In other words, it means trusting someone even if you are unsure if he/she is being honest. The speaker has used this expression in this video to accuse her sister of always being suspicious and doubtful of their mother, instead of trusting her. Ex: The employee said she was late because of a traffic jam, so her boss gave her the benefit of the doubt. Ex: Give me the benefit of the doubt. I have never lied to you.