The respective U(S)ART pin is led out directly without a level shifter. Therefore, the signal level has nothing to do with the NUCLEO itself, but with the STM32 used.
Now to the individual points:
If you supply the NUCLEO via E5V, the internal VDD of normally 3.3V is generated, which supplies both the ST-LINK and the target STM32.
All STM32s have a maximum supply voltage of 3.6V, so they can only output 5V levels if the respective pin is able and set to operate as an open drain, without internal pull resistors and with external pull-up to 5V.
Does it answer your question?
Regards
/Peter
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