What's the output of ICs if only one source pin is connected ( Vcc or ground)
This depends on implementation detail but in general it is a prohibited condition.
Modern CMOS-based technologies are extremely fragile and easily damaged by electrostatic discharge.  To attempt to protect the internals against this, I/O pins are traditionally protected against over/under voltage by diodes shunting out of range inputs to the supply rails.
But the diodes are not sized for continuous usage, hence application of a voltage more than a diode drop outside of the supply range to an I/O pin is typically prohibited; when the supply rail is unpowered that become a prohibition of having any I/O above a half volt or so.  Exceptions would be where it is explicitly allowed (eg some kinds of &quot;5v tolerant pins&quot;), or allowed up to some limit of injection current (for example pins that the manufacturer explicitly says can be &quot;5v tolerant&quot; with a suitable series resistor).
In addition to being a specification violation, attempting to apply a voltage without a supply will typically result in the chip trying to run off the I/O voltage, through the protection diode, and even backpowering the supply net on the board.  This tends to load the signal beyond the point of meaning, and may also put the chip or board in an indeterminate state from which normal power-on reset may not work.
The short version:  don't do that.