Why and how voltage is very high and current is very low in power transmission lines

In electric power transmission the voltage is stepped up to reduce the loss of electric energy due to Joule heating of the transmission wire.
Since, electric power,
P = VI.
So, the current will be smaller if the voltage is increased.

Energy dissipated as heat = i²Rt
Thus, low current transmission lowers the energy dissipated due to joule heating.