How would spacecraft protect against electromagnetic radiation?

Ultimately the electromagnatic radiation in space isn't that much of a threat. It is higher than on the surface of Earth, most certainly, but not massively. The multiple layers used in spacecraft construction will have some attenuating effect. Particle radiation from the solar wind and from solar particle events is far more dangerous.

In general, metals.

The thickness depends on the frequency of the wave: the higher the frequency, the thicker the material.

The denser the metal, the thinner it needs to be.

In practice, most of the electromagnetic radiation-sensitive components in a spacecraft are in Al boxes.

The real problem is energetic particles (high energy electrons and protons) and galactic gamma rays (GGR).

For the first two, shielding is typically done with Ta, when the Al box thickness is not sufficient.

There is no protection for GGR, the energy is just too high to shield with a reasonable amount of material.

On the other hand, the density of GGRs is low and they do not present a problem.

Gold shielding.

just what was used when we went to the moon and back several times? or was those trips fake???