Does a POD on someones accounts belong to the estate?

If your mother named you as the POD beneficiary on an account, that account belongs to you, and it does not become part of your mother's probate estate. It DOES become part of her estate for estate tax purposes, so if your mother's estate was large enough that an estate tax return is required, or if she was married and the Executor wants to allow her surviving spouse to use her remaining estate tax exemption and so an estate tax return is needed, then the Executor would need information about what was held in any such account at the date of your mother's death. But the Executor is not entitled to the funds. They're yours.

POD accounts pass outside the probate estate.  However, in most states, if there are insufficient funds in the probate estate to pay the estate's debts, people who receive accounts POD must pay those debts pro rata.