From Bulbapedia, the community-driven Pokémon encyclopedia.
A Pokémon's base friendship is the value to which its friendship is set when the Pokémon is caught, received from an event or NPC, or received from a trade with another player. If a Pokémon is caught in a Friend Ball, however, it will always start out with 200 friendship. If a Pokémon hatched from an Egg, it will start with 120 friendship.
Beginning in Generation III, different species of Pokémon were given the ability to have different base friendship values. Prior to this, Red's Pikachu had a base friendship of 90 in Pokémon Yellow and all Pokémon had a base friendship of 70 in Generation II.
In Generation VIII, the base friendship values of all Pokémon that had a base friendship of 70 were lowered to 50, except for non-Galarian Stunfisk and Timburr. Hoopa Unbound's base friendship was also lowered from 100 to 50, though this has no practical effect since Hoopa cannot be obtained in Pokémon Sword and Shield.
In Pokémon Black 2 and White 2, N's Pokémon start with maximum friendship. Starting in Generation VI, if a Pokémon is traded back to its Original Trainer, it will remember the friendship it had with that Trainer before being traded away.
List of Pokémon by base friendship