I will pummel the knowledge of everything up to Generation IX into everyone's brains if it's the last thing I do!
But yeah, it can get tricky--
Like.
Fighting/Steel. We have three Pokémon with this combo (Lucario, Cobalion, Crowned Shield Zamazenta).
Weak to Fighting, Ground, and Fire.
Neutral to Flying, Ghost, Water, Electric, Psychic, and Fairy.
Resistant to Normal, Steel, Grass, Ice, Dragon and Dark.
Very resistant to Rock and Bug.
Immune to Poison.
Ground/Fighting. We only have one Pokémon with this combo (Great Tusk).
Weak to Flying, Water, Grass, Psychic, Ice, and Fairy.
Neutral to Normal, Fighting, Ground, Ghost, Steel, Fire, and Dragon.
Resistant to Poison, Bug, and Dark.
Very resistant to Rock.
Immune to Electric.
Ground/Steel. We have six Pokémon with this combo (Alolan Diglett, Alolan Dugtrio, Steelix, Excadrill, Galarian Stunfisk, Iron Treads).
Weak to Fighting, Ground, Fire and Water.
Neutral to Ghost, Grass, Ice, and Dark.
Resistant to Normal, Flying, Bug, Steel, Psychic, and Fairy.
Very resistant to Rock.
Immune to Electric and Poison.
Fighting/Ground/Steel. There's currently no triple-types in the games (officially) unless you use Trick-Or-Treat (Ghost) or Forest's Curse (Grass), so this combo is currently not possible.
Weak to Ground, Fire and Water (×2).
Neutral to Flying, Ghost, Grass, Psychic, Ice and Fairy (×1).
Resistant to Normal, Steel and Dragon (×½).
Very resistant to Bug (×¼).
Highly resistant to Rock (×⅛).
Immune to Poison and Electric (×0).