I didn't know how to phrase the question in the form of a question. My apologies in advanced.
But in summary/example, I was trying to use a fighting type move on a Sableye. Normally, fighting types would give 100% immunity to Sableye. However, that only takes into consideration Sableye's ghost type. If a fighting move were to land on Sableye due to Foresight or otherwise, fighting type moves would be super effective. This is a given.
But in an inverse battle, shouldn't full immunity disregard its other type if its first type gives full immunity in a normal battle? Why would fighting type moves be normal damage to Sableye if Foresight was not used? I understand (and presume) that if Foresight were used in an inverse battle against Sableye, and then a fighting type move was used, then it would be not very effective, but the fact that it wasn't should mean that it gives 2x damage. Is my logic wrong?
Further examples would be earth attacks on Flying/Electric, Flying/Rock, Flying/Fire types, or dragon attacks on Fairy/Dragon types, or psychic attacks on Dark/Poison, Dark/Fighting types, or poison attacks on Steel/Fairy, Steel/Grass types, or fighting attacks on Ghost/Steel, Ghost/Rock, Ghost/Normal, Ghost/Ice types.