Devices are objects or systems that have a specific purpose or intention, like electronic communication devices like cell phones. But in the phrase "left to your own devices", it means your ability to figure things out alone or entertain yourself.

The noun device comes from the Old French word devis, meaning “division,” “separation,” “wish,” or “desire.” That original meaning is only retained in the expression “left to your own devices,” meaning "you're on your own." Otherwise, devices are things that perform a specific function, like tools, instruments or appliances, like a coffee maker, a devise that turns water and grounds into piping hot coffee.

Definitions of devices
  1. noun
    an inclination or desire; used in the plural in the phrase `left to your own devices'
    “eventually the family left the house to the devices of this malevolent force”
    “the children were left to their own devices
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    type of:
    disposition, inclination, tendency
    an attitude of mind especially one that favors one alternative over others
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