this may be a Mojave vs Catalina variation (or I suppose it could be laptop vs desktop Mac), but I did not have to turn bluetooth off. If the Magic keyboard/trackpad/mouse is connected via Bluetooth, when it is plugged in there is a short pause and a notification pops up that the device has been disconnected, which apparently refers only to the bluetooth connection as the device works normally. If the device does not have Bluetooth connection and it is plugged in there is no notification but the device simply starts working. The Magic keyboard/trackpad/mouse must be switched ON to work in either wired or wireless mode.
I don't know if this is a Mojave vs Catalina thing or not , but in Catalina when you select a device on the Menu Bar Bluetooth dropdown menu, not only does it show the remaining battery percentage as freelance said, it also offers options to Connect/Disconnect that device and to open the Bluetooth Preference pane.
SIDEBAR:
I solved my previously mentioned bluetooth propagation issue by standing the Mac mini on edge rather than sitting flat on the desktop next to the monitor stand which was apparently attenuating or distorting the Bluetooth signal propagation. As a side benefit that freed up more space on my desk. ðŸ‘
