A fixed temperature is going to act as an infinite source/sink of heat (depending on the temperature of the surroundings). As the name suggests, the temperature will be fixed to the value that you set no matter what is happening in the domain.
Of course this depends on your physics, but normally you’d see temperature definitions like this, which uses Newton’s law of cooling.
The electrical potential BC is just a reference potential. If you have a single current in, you’d normally go for current in + electric potential on the face where the current goes out.
Cheers