sangwoo

The best way I can describe it is like this:
TSV = Trainer Shiny Value

Every save file in the new gens has a special 4 digit number assigned to the trainer and every egg also has a 4 digit number called an Esv (Egg Shiny Value) if an Egg matches your Tsv it'll hatch as a shiny.

I'm sure there are other places/people who explain it way better though but trust me they're super helpful.

You're welcome.

To avoid a double post, not for any legendaries but there's a pretty great way to get a Shiny Rowlet if you know your Tsv and make a request in this thread.

https://www.pokecommunity.com/showthread.php?t=393126