When it comes to federal taxes, understand that "how" or "why" you have custody of a minor doesn't matter to the IRS. If that minor is a blood relative they have to qualify under the "qualifying child dependent" rules of the IRS for you to claim them as your dependent. Just because you may have been awarded custody by the court, doesn't mean they will qualify as your dependent.
If the minor is a non-related child, then they have to qualify under the "qualifying relative dependent" rules of the IRS. They don't have to be an actual "relative" per-se. But those are the rules which apply to a non-blood related minor you wish to claim as a dependent on your tax return.
For a "plain english" definition of the rules see https://www.1040.com/tax-guide/taxes-for-families/who-can-you-claim/
