I'm gonna try to simplify it.
Imagine every Pokémon having a hidden unchangeable value, and that value and range from 0 to 4095.
In GBA/NDS games, if a Pokémon has any value between 0 to 7, it would be shiny.
Those methods you mentioned don't change the value the Pokémon can have. It simply expands the shiny limit of 0 - 7, to let's say, 0 - 255.
So let's imagine we have a Pokémon with value 255.
In the modified game, it appears shiny.
However, if you transfer or trade the Pokémon back to any non-modified game, the Pokémon will be (correctly) non-shiny, as the shiny range is only from 0-7.
[255 > 7]
Does that explanation help?
For modifying Gen 8 saves directly, yes you need a modified Switch