I'm a nerd so I can give some more clarification
It - used for anything without a gender, or unknown gender, except for people. It includes animals, objects, and ideas. Using "it" for a person implies they are less than human or not meeting their gender roles and can be used as an insult. Think of kids making fun of a really ugly or fat boy/girl and calling them an it.
They - Recently has been used more often as a singular neutral pronoun when gender is unknown. But yes has some relations to snowflake genders because of people who insist on the "they" pronoun because they believe they have no gender.
He or she - What is supposed to be said when gender is unknown and why the shorter "they" gained traction.
One - Used for unspecified gender, but not usually for unknown gender
To clarify, you can say "one would usually like to eat every day" because you're not taking about someone specific, but rather people in general. So it's used for metaphors and such.
But you can't say "One is wearing a dress today" because it refers to a specific person.