I have many nodes.
Each node consists of an MPU9250 and ESP32. Each node functions independently. They don't interact with each other.
How can I power all of them at the same time, and switch them all on / off at the same time easily ?
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You need to provide more details. How many is "many"? How far apart are they physically? Can you run wires between them? Could you use radio control?
You said in the comments that they are each powered by separate batteries.
One possibility would be to put a power MOSFET transistor on each battery, switching the power supply for a given node. Then wire the gates (and grounds) all together and control them with a switch connected to the supply from one of your batteries (or from a 5V logic line.) MOSFETs draw almost no power at idle, and draw very little gate current for switching, so you should be able to drive dozens of them from a single 5V control line. However, this would require that you have a wire running from a central switch to every node, as well as a common ground to all of them.