Both the Accessory Power Socket (once known as a cigarette lighter socket) and the USB A are kept active with the SE switched off.
I have not checked but I believe the Accessory Power socket in the hatch area is also always powered.
I expect this is to ensure that the flat tyre compressor kit can be used with out requiring the Radio Ready setting.
I would also note that the USB A socket in the SE has a lower power output than the USB C socket (where installed).
You may want to find a device that plugs into the 12 Accessory socket that provides USB C that could benefit from higher amperage output.
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Ok that makes senseI bought one for my wife’s i3 as it also only has USB A sockets. I bought an Anker Powerdrive Speed+ from Amazon for £30 which has a USB A port allowing up to 19.5W and a USB C PD port allowing up to 30W charging for fast charging our iPad Pro which we sometimes take with us on trips in case we have a car charging stop, and fast charging her iPhone as she’s always running out of charge in her phone. Anker are one of the few aftermarket products I’d use with my electronic devices. Not the cheapest but the quality has been good and I’m confident it won’t burst into flames.
On the i3, the accessory socket stays powered for a short time after leaving the car but does switch off. It’s my understanding that the Mini does the same; hence the need to hardwire a dashcam to the fuse box if you want it to monitor the car while parked. Plugging it into the accessory socket doesn’t allow that.