It is really very simple:
When you close the switch the electric field between the wires travels almost at the speed of light towards the load. When it reaches the load a current starts to flow.
This current travels back as a magnetic field between the wires. When the current reaches the load the voltage drops. This voltage drop travels towards the the load. When the voltage drop reaches the load the current drops. This current drop travels back to the source.
This process repeat, almost at the speed of light, until it becomes stable.
In this process some of electromagnetic field 'leaks' into the wires, heating up the wires, and some of the electromagnetic field 'leaks' into the isolation, heating up the isolation.
...and this is the simple explanation.
James Clerk Maxwell described this in exact mathematical details in 1865.