Ive looked at game instances and save game to slot but i dont really understand how id apply it to the game once player hits play in the ui
Load that stuff in game instance, as this thing starts first.
Then set all variables you need that you just loaded.
Bet places to save variables:
ini file,
json file (great for storing for eg quest text etc),
or data assets (this is best if you for eg have two or more different setups that you are still testing) lie first person 3rd person camera etc.