What is the commonality between a radio, a welding power source and an induction heater?
The basic concept here is probably "resonance", which is related to the ringing of a bell or the swinging of a pendulum at its natural frequency.

Strike a bell and it will ring. But if you play back the sound of its ringing, it will also start to ring. This is because the resonant frequency is optimal for it accepting energy.

You can keep a swing going over a large range of motion with a small push, correctly timed. Same phenomenon.

LC circuits also "ring". In the case of radios, this is used for tuning: it resonates most strongly near the desired radio frequency. So it absorbs radio waves at that frequency and turns them into an electrical signal, while absorbing waves at different frequency less strongly.

In the case of the heater and welder, it's being used to optimise the power transfer through a transformer to produce a large current.

A full theoretical understanding of AC involves calculus and complex numbers.