Pokémon Type Effectiveness Guide
All Pokémon are assigned types. Types affect various characteristics of a Pokémon, including effectiveness in battle against other Pokémon, the type of moves it can learn, design features, and often the color palette chosen for that Pokémon.
Currently there are 18 available types a Pokémon could have. All Pokémon belong to either one or two of these type categories. 
Charmander, for example, is a mono Fire type, while 
Bulbasaur is a dual Grass and Poison type. Types are consistent across species, meaning all Charmander will be Fire types. Let’s take a closer look at how Pokémon types affect battles.
How To Calculate Type Effectiveness
Like Pokémon, attacks are also assigned types. For instance, 
Blaze Kick is a Fire type attack. When a Pokémon uses an attack, the type of the attack is compared against the type of the defending Pokémon. Depending on the type matchup, the attack could be neutral (100% damage), super effective (200% damage), not very effective (50% damage), or have no effect (0% damage). Knowing the effectiveness of attack types on Pokémon types can give you a huge advantage in battle!
Check out Dittobase’s Pokémon Type Chart to see how all 18 types interact with each other!
Calculating Dual Type Effectiveness
Attack effectiveness is multiplied for dual type Pokémon. For example, 
Gyarados, a dual Water/Flying type, would take 400% damage from an Electric attack, since Electric is both 2x effective against Water and 2x effective against Flying. This works the same for resistances. 
Scizor, a Bug/Steel Pokémon, would only take 25% damage (.5 x .5) from a Grass type attack, since Grass is not very effective against both Bug and Steel.
To easily calculate dual type effectiveness, you can also use Dittobase’s Pokémon Type Calculator. This useful tool allows you to select your Pokémon's type(s) to see how well it defends against each attack type, as well as checking offensive attack effectiveness and coverage.
Same Type Attack Bonus (STAB)
Another factor to consider when calculating attack damage is Same Type Attack Bonus, abbreviated as STAB. If a Pokémon's type matches its attack type, the attack will do an additional 50% damage.
For example, say we have a 
Blastoise that knows 
Hydro Pump. Hydro Pump is a base 110 power Water type attack. Since Blastoise is a water type Pokémon using Hydro Pump, a water type attack, the STAB bonus applies, essentially bumping the power of Hydro Pump from 110 to 165 (110 x 1.5). Because STAB and type effectiveness are multiplied, your Pokémon has the capability of doing massive damage against the right opponent. A mindful trainer will consider this and make the appropriate switches in battle.
Tera Types (Scarlet & Violet Only)
Tera types, introduced in Pokémon Scarlet and Violet, add another dimension to Pokémon typing. By using a Tera Orb in battle, you can Terastallize your Pokémon, switching its type completely! When a Pokémon Terastallizes in battle, it loses its original type(s) and becomes whatever its Tera type is. The Tera type is listed on the Pokémon Status Summary screen, directly under its type.
How Tera Types Work
Let's examine this 
Miraidon. Miraidon is an Electric/Dragon type. This particular Miraidon has Electric for its Tera type, meaning when it Terastallizes, it loses its Dragon typing and becomes a pure Electric type. This means Miraidon will no longer have its weaknesses to Fairy and Dragon type attacks but retains its weakness to Ground type attacks. Terastallizing into one of the Pokémon's original typings will also boost the STAB bonus from 50% to 100%, meaning even in a neutral type matchup, Miraidon will be hitting for twice the damage when using Electric type attacks! This is a great example of how to use Terastallization to boost your offensive power.
Terastalliztion is also often used as a defensive tactic. This shiny Scizor, for example, has a Bug/Steel typing, with a Tera type of Water. Normally, Scizor would take 400% damage from Fire attacks since both its typings are weak to Fire. However, if you predict an incoming Fire attack, you can Terastallize into the Water type to reduce that damage down to 50%. Additionally, a Pokémon will gain STAB on attack types that match its Tera type, while retaining STAB on attacks of its original type(s). This means that after Terastallizing, any Bug, Steel, or Water type moves Scizor uses will do an additional 50% damage!
If a Pokémon can’t learn any moves that match its Tera type, you can always teach it TM 171 Tera Blast. Tera Blast is a 80 BP attack that will always match the typing of your Tera type. It will deal either Physical or Special damage, depending on whichever stat is higher of the Pokémon using it: Attack, or Special Attack, respectively.
How to Change your Pokémon's Tera Type
Most wild Pokémon will have a Tera type that matches one of their standard types. However, certain glowing Pokémon in the wild and Tera Raid Pokémon may have different Tera types. The good news is Tera types can be changed!
To change your Pokémon's Tera type, you must first beat the Normal type gym in Medali. Next, collect 50 Tera Shards of your desired type. Shards can be won from defeating Tera Raid Dens, or from completing classes at the Academy. When you have the required number of shards, head to the Treasury Eatery in Medali and speak to the Chef behind the counter. He will offer to change your Pokémon's Tera type in exchange for 50 Tera Shards. Think carefully before choosing your Tera type, as these shards can be hard to come by.
Stellar Tera Type
The Stellar type, introduced in the Scarlet and Violet Indigo Disk DLC, is worth a special mention. Below are the key points to remember about the Stellar type:
- Currently, no Pokémon have the Stellar type as one of their main types. This type is only accessible as a Tera type when Stellar Tera Shards are used to change your Pokémon's Tera type to Stellar
- There are no Stellar type moves, however, the moves Tera Blastand Tera Starstorm become the Stellar type if the user has Terastallized into the Stellar type
- Stellar type moves are super effective only against Terastallized Pokémon
- Pokémon that Terastallize into the Stellar type maintain the weaknesses of their original types
Conclusion
Hopefully this guide helped you understand how Pokémon types interact with attack types and provided you with useful tools and information to become a better Pokémon trainer!