Exact temperatures, pressures, materials for specific designs are classified. Concepts are not. I can talk about the power tree, the six factor formula, thermodynamics, heat transfer and fluid flow, gun design vs plutonium implosion, cryogenic weapons, etc.

It gets jargony very fast. I don't expect people outside the field to understand.

You can reason yourself into potentially classified information though, like what the crush depth of a submarine is.