All right, after doing a little digging I believe I have found the correct answer to this question.
According to Bulbapedia:
A Wanderer is a type of wild Pokemon encounter found in Pokemon Sword and Shield. It is a wild Pokemon that appears in a fixed location in the overworld and which may react to the player's presence in the same manner as other symbol encounters.
Before I add anything to Bulbapedia's definition, a symbol encounter is the type of wild Pokemon encounter that was introduced in the Let's Go, Pikachu/Eevee! games. Instead of having to walk through a patch of tall grass or a cave to encounter a wild Pokemon, as in older games, in the Let's Go! games all the wild polemon spawn from tall grass, in caves, etc. and walk around in the overworld. You can actually see a Rattata dart from a patch of tall grass or a Geodude appear and float around in a cave, for example.
Well, a "wanderer" encounter is a unique type of symbol encounter. Instead of spawning from tall grass or random areas in caves, bodies of water, etc., a wandering Pokemon has a specific area in which it always spawns. In the Wild Area of Sword and Shield, a wanderer spawns once a day.
For example, in the Wild Area, there is a wild Duraludon that you can always encounter at the same area of the Giant's Seat. If you either cause the Duraludon to faint in battle, catch it, or flee from it in battle, the Duraludon will despawn. But if you return to the Giant's Seat the next day (starting at 12:00 a.m.), you will find another Duraludon stomping around in the very same area as the previous one.
Duraludon isn't the only example of wandering Pokemon in the Wild Area. The wild Onix in the rolling fields, the Snorlax at the Bridge Field, and the Obstagoon near the Wild Area's Pokemon Nursery are other examples, and the same is true for them as for the Duraludon at the Giant's Seat, as well as for all wandering Pokemon in the Wild Area.
There are also wandering Pokemon in the Isle of Armor and the Crown Tundra. All that is true for wanderers in the Wild Area is also true for wanderers in these areas.
Wandering Pokemon can also be found in other areas throughout the game. These Pokemon are:
- A Chewtle on Route 2
- A Yamper on Route 2
- A Drednaw on Route 2, on the island in the middle of the lake
- An Obstagoon on a hidden path of Route 2
- A Lapras on the lake of Route 2
- A Corvisquire perched on Route 3
- A Carkol in the first Galar mine
- A Woobat in the first Galar mine
- A Crawdaunt at Turrfield
- A Diglett on Route 4
- An Eldegoss on the bridge on Route 5, drifting down from above
- Several Galarian Stunfisk at Galar mine 2
- A Drednaw st Galar mine 2
- A Gastrodon at Galar mine 2
- An Impidimp at Glimwood Tangle
- A Morgrem at Glimwood Tangle
- A Crustle on Route 8
- All the Falinks that appear from the holes in the ruins on Route 8
- A Glalie on Route 9
- Several Grapploct on Route 9
- An Abomasnow on Route 10
- A Beartic on Route 10
Note that whether the listed Pokemon respawn or not and when they respawn varies. Some respawn if you exit the area and then return, some respawn daily, and some never respawn.
Sources:
Bulbapedia
Serebii
I hope this finally answers your question!
