Once Pokémon spawn around you in the overworld, their stats are already determined.
The seed determines PID, IVs, EC, Height, Weight, and this viewer converts the seed to those properties.
This is just a viewer, not an editor.
It reads the kcoordinates block exported from PKHeX.
You're also gonna need to download this file, specifically the 32-bit version: https://www.microsoft.com/en-us/download/details.aspx?id=13255
PKHeX's Source code in relation to Xoroshiro calculation and Stats determination from the seed.