The word everyone refers to all people considered together or to all members of a specific group without exception.
“You use everyone to refer to all the people in a particular group or all people in general.” — Collins Dictionary
Everyone is a singular indefinite pronoun grammatically, so it takes a singular verb form:
Correct: Everyone is ready.
Incorrect: Everyone are ready.
Author’s summary: The word everyone means all people taken together or every person in a group and is grammatically singular in English.