I don't know where the default icons are stored, possibly even inside the launcher's executable file or a .dll file (it is possible to edit assets within these files but the next launcher update will remove them, possibly even the next launch if it performs integrity checks on itself, as is done for the actual game files, which is why mods need to use custom version jars) but it is possible to add your own icons by embedding them inside the launcher_profiles.json file as base64 encoded images (a long string of random-looking text):
(this video is a bit old and the launcher now looks different but a comment said it still works as of 8/24/2020, after the most recent reskin, and I see the same entries that they showed in launcher_profiles.json; obviously, always make a backup before editing it)
correction: the custom icons (like optifine, labymod, forge and the other ones that are custom) are located in launcher_profiles.json as a inlined png encoded in base64, and the preset icons like the furnace grass block and the other ones that are preset in the launcher are located in C:/Program Files (x86)/Minecraft/game/launcher.dll/launcher/assets/icons/
anyone know where'd i find the crafting table icon that is shown in the dock after minecraft is launched? ive looked for a while in the minecraft jar and the minecraft folder and couldn't find it. (im using manymc) thanks.