To increase your chances of finding a Shiny Pokémon, you’ll want to chain battle the Pokémon in the mass outbreak. The more Pokémon you defeat, the higher your chances of finding a Shiny Pokémon.

Even without the use of sandwiches or the Shiny Charm, if you defeat 30 to 59 of the same Pokémon, your chances of encountering a Shiny double, whereas if you defeat more than 60, your chances will triple.

Couple this with a sandwich or the Shiny charm, your chances will continue to increase.

Source: https://www.distractify.com/p/how-to-shiny-hunt-pokemon-scarlet-violet