Here is a site with a good explanation.
https://www.fidelity.com/viewpoints/non-spouse-IRA
My wife inherited her Father's IRA. We used the IRS table for her life expectancy and started the withdrawals (RMD) the year after her Father died. One important note was that he hadn't taken his RMD at the time of his death for that year. So we took it on his behalf and filed his final tax return. So you don't start distributions until the year after the person's death.