A note could be a card or an email, but a thank-you note simply expresses gratitude for the interviewer’s time and attention.
And moreover, a followup will keep your name in their mind, for better or worse. Following up after the interview series may have been the trick to landing my “pandemic job” in May 2020. I phoned or emailed the recruiter, just to thank her, just to say hello, and you know what? She said “yeah actually we were gonna call you with a job offer” and the rest is history.
So, before you complain that someone ghosted you, please consider who’s ghosting whom!