In BKK there's a reentry permit desk to the left side. In DMK there's an office before you get to the immigration desks. If you can't find it, tell an immigration officer BEFORE you hand them your passport, they'll direct you to the desk.
Assuming this is a legitimate question, your visa starts the day you enter the country, not when you finish quarantine. Also, there is no more 14 day quarantine.
And yes, many people have left the country and reentered after having Covid extensions.