I've built vacuum tube amps with 600+v and the circuit is essentially isolated using input and output transformers which I see as a complete isolation.
There should also be a power transformer to isolate the power line voltage from the circuitry (and to isolate the user).
Transformers do provide 100% electrical isolation. There is NO ELECTRICAL CONNECTION between the primary and secondary. Only a magnetic connection.
The power supply in just about every piece of electronic equipment has a transformer to isolate the power line voltage.
There is often a common ground. For example, in your (desktop/tower) computer the power supply output-ground (which is connected to the motherboard ground) is connected to the power-line & chassis ground.
I think the main purpose of the input/output transformers is impedance matching. I believe there are tube preamps without I/O transformers or with input transformers but no output transformer. The output transformer in a tube audio power amp converts a high voltage, low current signal into a lower voltage, higher current, signal to drive a (low impedance) speaker. From what I understand, some high fidelity tube amps use feedback from the transformer output to reduce transformer distortion and to improve frequency response. That means the audio output is NOT 100% electrically isolated from the tube-voltage.
On the input of a tube amp, a step-up transformer can give you "free voltage gain" since the input (grid) on a tube has very high impedance.
Transformers can transfer power. A transformer is like the transmission in a car or the gears & chain on a bicycle.
Relays & opto-isolators only control power, like the gas pedal in your car.