Questions about ability checks and multiclassing with spells.

[JNAProductions]

I get that ability check is used in trying to see through an illusion, but what about preventing one of your own illusions from being seen through? The value you have to oppose the opponents investigation skill for your illusion isn't an attack or a save as far as I can tell. So won't either or both of these abilities help with making ones illusions more reliable? Please explain illusion magic check for those who cast the illusion (not for those opposing and trying to see through it)

It uses your spell DC.

Your spell DC is 8+your proficiency modifier+your spellcasting stat modifier.

It is not an ability check, so Jack Of All Trades or Reliable Talent never enter the picture.