Terastallize is Pokémon Scarlet & Violet’s regional gimmick, a new mechanic they introduce with each new generation that generally doesn’t return from game to game. Unlike Mega Evolutions or Alpha Pokémon, Tera-types have a massive impact on strategy in Pokémon battles. Let’s take a look at how they work.
Related: How do Tera types work in Pokemon Scarlet and Violet? answered
How do Tera types work in Pokemon Scarlet and Violet?
Each new Pokemon can be assigned a Tera-type, so your Pikachu could get a fighting Tera-type. That is, when it terastallizes, it transforms from an electric type to a fighting type. It does not retain its original base type. A Pikachu with this fighting Tera-Type doesn’t morph into an Electric/Fighting-Type, it morphs from an Electric to a Fighting-Type.
If you catch a Pokemon with a specific Tera type, your Pokemon won’t get stuck on that one Tera type. Collecting Tera Shards allows you to change your Pokemon’s Tera type to any of the eighteen types. You can find these shards by completing Tera Raid battles or completing courses at the Academy. If you want to catch Tera-type Pokemon, they can be found glowing in the wild. Just be careful because they are usually stronger than most wild Pokemon.
Your Pokemon that doesn’t keep its original type can be beneficial because it can gain STAB (same-type attack bonus) from its new Tera-type as long as it has the same type of attacks as its new Tera-type. So if your Pikachu knew Mega Kick with its fighting Tera-Type, it would change from an Electric-Type to a Fighting-Type and gain STAB when using Mega Kick.
If your Pokemon is the same Tera-type as its original type, like a Pikachu with an Electric Tera-type, then it gains twice as much STAB, which is 100% more damage than usual.
The Terastallation can only happen once before you have to go to a Pokemon Center and charge it up, so it can’t be made infinite. Be careful where and when you terastalize.
https://www.gamepur.com/guides/do-tera-types-replace-a-pokemons-original-types-in-pokemon-scarlet-and-violet Do Tera types replace a Pokemon’s original types in Pokemon Scarlet and Violet?