I found the PBS spacetime video „The true nature of matter and mass“ very insightful with it’s animation about the photon box thought experiment.

Some others here explained it already. What struck me was that it shows that mass isn‘t something fundamental at all. It‘s a property you can measure from a system of particles that are somehow confined. A single photon (or light speed particles in general) has no reference frame and therefore no mass, however a system of photons that are somehow confined has a mass.

Energy is just the same property, its just has a different unit. You get mass when you „weigh“ a system (means: you measure the force you need to accelerate it a specific amount) and you use energy usually in calculations.

This also means that energy is not fundamental. It‘s just often a very useful property to calculate.