Company B has replied to my email asking me where I accepted an offer,
should I tell them?
There's no benefit to you from informing Company B of where you will be employed.
While it's unlikely to have any adverse affect either, there's a small possibility that someone at Company B will try to undermine your acceptance (call company A and say bad things about you, for example). I personally don't know a case where that has actually happened, but I've heard stories.
There's probably nothing nefarious going on here. Company B probably just wants to stay in touch. But after you are happily settled in your new position, you could reach out and contact them if you choose, with no risk.
For now, I think just letting Company B know you have accepted another offer, thanking them for their time, and wishing them continued success is the most prudent course of action. That's what I've always done in similar circumstances.