I cannot find a tutorial anywhere, I wish to make some foods that do not have diminishing returns, could someone provide an example for me? (both json and how the config would look in order to enable), what should I put into "food groups" for it to see my json?

Saving this as test-blacklist.json in config/SpiceOfLife should exclude apples from diminishing returns (no changes to SpiceOfLife.cfg needed):

{
	"food": {
		"items": [
			"minecraft:apple"
		]
	},

	"blacklist": true
}

See config/SpiceOfLife/example-food-group.json for more documentation.