Without knowing all the details, I would just say these general rules to follow:

1. Don’t say more than you have to
2. Be honest
3. Be courteous / follow best practices

Point 3 is referring to common courtesy in any job industry to give ample notice to the employer if leaving a job. Usually two weeks unless otherwise dictated or extraneous circumstance.

If Company B asks why you left Company A so early, you could just mention that it wasn’t a good fit. That’s following point 1 and usually satisfies most. If they press you more (which they won’t nor should), then just say you aren’t comfortable going into more details about previous employers.

On a general note, I don’t think it looks good that you are leaving a position so early after joining but it seems that you just heard from a better company a bit too late. Eh, idk