What is the secret into getting candles out of metal containers?

Are you asking how to get a candle out of a metal candle mold? https://www.wikihow.com/Remove-a-Candle-from-a-Mold

Your freezer..... Place containers in the freezer for a couple of hours, then turn them upside down on a towel and tap them. The wax should pop out. This works best with candles that have a high oil content.

Ditto on what Laura said ♥

Agree with the above poster.

If the frozen candles don’t pop out and you can’t shatter them with a heated ice pick, try putting only an inch of water in a shallow boiler. when the water is boiling, set the metal holders in the pot. Turn off the stove. In a minute or two the candle wax should be melted enough to pour out onto paper towels or another vessel. You may have to heat the candleholders a second time to get the rest of the wax out. Then wash and hand dry the candleholders to make sure they are clean. ☺️