How to power STM32F103C8T6 in a 'real' project?
I see several options:


Use a USB cable and a charger with a USB outlet and connect it to the USB port of your blue pill.
Cut a USB cable in half and solder the red and black wires to the 5V and GND connections of your blue pill, respectively. The blue pill has a voltage regulator to generate 3.3V. Then plug the USB cable into any charger with a USB outlet.
Buy a small power supply that outputs either 5V or 3.3V and connect it to the 5V or 3.3V connector and to the ground of your blue pill. Either solder the connection or - if you have a 3.3V supply - connect it to the 3.3V and GND pins of the ST Link connector.
Do the same as above with a power supply generating a higher voltage and additionally use a buck converter to generate 3.3V.