Isolators lead very very little current - hence will the Resistance be high.

Semiconductors (pure) will lead a little current and could be consired an Isolator.
With the right kind of doping will a Semiconductor eigther be a P-type or N-type.
Doped semiconductors could be considered as Conductors.

Conductors normally be metals like copper or aluminium with lots of free electons. The resistance of a conducter (wire) will normally be very low - hence will the power lost be little.

Resistors made with carbon-film at a glass-tube. They will be trimmed to have a specific value and used in circuit to reduce / control the current-flow.

Resistors in Series could be replaced by the sum of the Resistances.

Resistors in Parallel could be replaced with a resistor = 1/( 1/Ra + 1/Rb + 1/Rc .....)

Two resistors in series divides the Voltage by the value of their resistance.
        Vx  = Vcc * R2 / (R1 + R2)

Two resistor in parallel divides the current by their value (the lowest value of resistance leads the biggest Current)