What to say when quitting a job you just started?

Ive never been in this situation but I have played out the scenario in my head - I would just go into my boss/HR and be like 'now that I have been on the job for a few days I realize this isnt going to be a good fit for me and I do not want to waste any of your time training me further so I think its best if I resign'

I have done this before, and it is hard to navigate those fears of feeling wrong or bad. The best reason to give, i have found, is something along the lines of, “Thank you very much for giving me this opportunity. Unfortunately, i am realizing that this job isn’t really a good match for me, so i am needing to resign and move on to something that better suits my skills and inclinations. How can i help make this transition smoother for you?” Sometimes, you won’t receive a positive response, even with your best intentions invested, though. When that happens, it’s important to remember that knowing yourself, and this act of self-care, is one of the most wonderful and noble things you can do in life. Hold on to that, even when faced with negative feedback and reaction. You can trust your own judgement, always. There’s a reason you are feeling something is ‘off’ even if it is a fairly new situation. You can do it!

I might say something like, Thank you for the opportunity you have given me, but I am unable to continue. It isn't a good fit, or I was offered another position that is closer to my skill set, etc. My last day will be.... There isn't necessarily a reason that has to be given for leaving a new job, but it can be helpful in some cases in my experience. Sometimes I haven't shared anything about why I'm leaving, because I left for ethical reasons and I need the referral even though it wasn't a good situation.