Who is 'they' indicating?

What's "how true of you" mean here?

Here, "how true of you" means that the statement "disguise is always a self-portrait" is true for her. It's accurate and correctly describes her in that she didn't try too hard to be someone else. If something is "true of you," it means that it represents you and your behaviour well. Ex: How true of you to always be positive. Ex: It's true of you to tell others to be kind because you are always kind.

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

There is an article before "sea" because they do not specify which sea it is. I do not think "sea of plastic" is considered a countable noun.

What's the difference between "equipment" and "gadget"?

A "gadget" is a small mechanical or electronic device, and It's usually smart and inventive. You see "gadgets" a lot in spy and secret agent movies. A "gadget" can fall under the category "equipment", as "equipment" can be various sizes and materials. Ex: What a fancy gadget to open the door with! Maybe I should get one. Ex: Can you get the rock-climbing equipment from the garage? Remember to bring the ropes!

What's she mean by "mean the best?"

"Mean [for] the best" has a meaning somewhat similar to "best wishes" or "good luck." Basically, it's an expression used when you want to convey your best intentions, in the hopes that one's behaviour or actions produce a good result. Ex: I didn't mean to hurt you. I meant the best. Ex: She meant for the best, but her actions unintentionally hurt many people.

I don't understand her intention of saying this sentence. Could you please explain?

In this script, "I know what makes you a boy" means "I know why you are a boy" or "I know what proves you are a boy." The little girl is expressing her opinion that the man's bald head is how she knows he is a boy.