Stellar Type

For the first time since Pokémon X & Y, a new type has been introduced but, rather than being a general type that Pokémon can be, this type is exclusive to when you Terstallize Pokémon. This is the Stellar type, introduced in The Indigo Disk.

Terapagos can only have the Stellar Tera Type while you can find wild Stellar Tera Type Pokémon throughout the Terarium, you can also change your Pokémon's Tera Type to Stellar-type by using 50 Tera Shards at the restaurant in Medali.

This type is a bit different to the other types you can Terastallise into in that it doesn't actually change the base weaknesses and resistances of your Pokémon. While your Pokémon are the Stellar type, their weaknesses and resistances are identical to how they are when you aren't Terstallized.

As such, the only changes to the Stellar Tera Type are in how the move damage is calculated.

What the Stellar type does is give a special boost to Same Type Attack Boost of your move. Every move of a type gets a small boost for the first time that type move is used by the Pokémon in battle. This is in addition to any STAB that the Pokémon has.

If a move doesn't match the Pokémon's base types it will get a 20% boost in the damage for the first time that move's type is used. So if a Galarian Zapdos uses Throat Chop, it'd get a 20% boost for that first usage but no extra boost for any other usage, and if it then went to go for Knock Off, Knock Off also wouldn't get the boost.
If a move matches the Pokémon's base type, then like standard Terastallization, it will get a 100% boost bringing it to double, much like when you Terastallize into the type the Pokémon already is. After the first use it will return to its standard 50% boost.

Terapagos is immune to these effects and gets the 20% damage boost with every move that it uses, regardless on if has been used already. It will also always have the 100% boost to Normal-type moves except Tera Starstorm.

The moves also always have the boost during Tera Raid Battles for all Pokémon.

There are two moves that are different to this, however. Tera Blast and Tera Starstorm both change their types into the Stellar Type when you Terastallise and, with that, they get added effects. However, they will only ever have the 20% damage boost and like the others, that boost only happens for the first usage.