This isn’t a good answer. On average in the US we lose 7% of power in transmission and 8% in power conversion. I pulled these numbers out of thin air, but it’s probably accurate to within a factor of 2 for wherever you are. A home solar power system might mean 7% lost in dc to ac and then you still lose 8% for conversion so basically 5-20% of all the power you produce is lost before it reaches its destination.
A more didactic answer might start with defining power, P=IV (power equals current times voltage) and “ohm’s law” V=IR (voltage equals current times resistance) and suggest this means that the power lost to the resistance of transmission line with resistance R is P=I^2R. Since the current you send is proportion to the voltage (One megawatt can be 1Amp at 1000000 Volts or 1000Amp at 1000 Volts, you want to use as high voltage as possible when transmitting long distances. The limiting factor on voltage is the amount of insulation required.
Usually power transmission uses three-phase alternating current, which sends power by varying the voltage on three lines each 120 degrees (1/3 period) offset from one another. It’s reasonable homework to read about why AC is better for transmission and also how three phase AC works and determine the economic value of laying three cables instead of two, with variables for the cost of wire, fixed costs per mile, and cost of electricity. ~50% of transmitted power is used to drive three-phase synchronous AC motors.
Resistance is related to the resistivity (denoted with a Greek rho) of the material and the cross-sectional area (denoted A) and length (L) of the wire: R=rhoL/A Resistivity is a fundamental property of materials and is related to the free movement of the outer shell of electrons of atoms in a crystal lattice.
Superconductors do not have resistance and can carry an extremely large current as a result. Unfortunately superconductors have a maximum magnetic field capacity and require extremely low temperatures requiring cooling with liquid helium or nitrogen.
