When connecting a serial device (for example an Arduino via USB or Bluetooth), the owner group: is defined as root: dialout, for example:
crw-rw ---- 1 root numbering 166, 0 April 15 18:08 / dev / ttyACM0
However, even when the normal user is part of the dial group, it is necessary to change the permissions on the device file (in 666).
This is only temporary until the device is disconnected. A persistent solution is to edit the udev file. See for example this link and this link
This can be confusing, especially when the Bluetooth system tool (blueDevil on KDE) can detect USB devices, but can not connect to them.
It seems to me that the default behavior should be to grant write access to users with a dialout or, at a minimum, that this should be part of the GUI system setup tools.