On mutual inductance of transformer!

First, the transformer coil ignores the resistance, but the inductance (L) cannot be ignored. After electrifying, it will produce a back electromotive force, ε=-di/dt, or it can also be understood as inductive reactance (middle school students seem to be not ready to talk about the obstacles to current in the circuit: resistance, inductive reactance and capacitive reactance are the functions of resistance, inductance and capacitance respectively), so that the current will not tend to infinity.

Second, yes, the voltage of the primary coil is controlled by the power supply, and its alternating current will produce a changing magnetic field. The magnetic induction lines of these magnetic fields pass through the secondary coil, and the magnetic flux of the secondary coil changes constantly, thus generating induced electromotive force.

Third, it involves the conservation of energy. You will find that there is a P=ui energy output in the power supply, and the primary coil is useless for energy loss. Then all the energy is given to the secondary coil, which means that the voltage across the secondary coil is determined by the power supply. There is no doubt that then the voltage of the secondary coil is determined, P' and P are also determined, and the current is also determined. Note that this is not a pure resistance circuit and cannot be explained by ohm's law, which we are used to. The essence of the problem is that I explained ε=-di/dt.

Note: If you haven't learned derivative yet, I'll tell you by the way that di/dt represents the rate of change of I with t, just like the induced electromotive force ε = dφ/dt (the rate of change of magnetic flux with time).

I hope it helps you.