What the FADEC do when we put the levers in TOGA?
It sets the power according to the temperature you have entered.
(A320 series). It sets full power. Every time. If you set the levers in the FLX/MCT gate, it will set the power to the reduced thrust level.
How the FADEC does knows the power that need to put, if we don’t put neither temperature?
You have to enter a temperature.
If you don't enter a temperature and set the levers to FLX/MCT, a warning is generated to set the levers to full thrust position (TOGA). The aircraft has sensors for the OAT, you don't have to enter it.
ps. Flex lower than the OAT, the FMGC gives an 'invalid entry' type warning and doesn't accept the entry. i.e. you can't do it.