A will has no effect on a life insurance policy, IRA, 401(k), or any other “non-probate asset” for which you have executed a proper beneficiary designation. You can even tell your bank how to pay out your account when you die.
If you fail to designate a beneficiary, however, then that asset will pay to your estate. If you divorce after designating a beneficiary, then that designation is nullified. The divorce also nullifies any prior provision in your will that names your ex-spouse as a beneficiary or executor. If you want your ex-spouse to remain a beneficiary (maybe the divorce decree requires it), you need to execute a new will and/or sign the appropriate insurance form again.
Non-probate assets immediately become the property of the beneficiary and are not available to pay your debts. So, if you name your brother as the beneficiary of your life insurance because you think he will use the money to settle your debts or support your kids, you may be making a big mistake. He can claim the money as his alone and leave the estate to pay those obligations out of any remaining probate assets.