If you are using Warcraft 3 Reforged, then you can't find all MPQ files because Reforged uses the CASC format.
All you need to do is use the Ladik's CascViewer. For the case of icons, take a look for the "ReplaceableTextures" folder.
There should be a two important folders called "CommandButtons" and "CommandButtonsDisabled". Each folder contains all files, with each had "BTN" and "DISBTN" prefixes.
For passive abilities, it should be "PASBTN" and "DISPASBTN".