For regular players they can enable the flags when creating a world for the features they want or they can use the existing minecraft command /datapack enable/disable

for regular players they need to go

config/corewithstuff-features.json

and make true or false the features they would like to have

those options will be overridden by an palladium addon or a mod if their pack.mcmeta include the flags

for developers they need to use the flags they require in the pack.mcmeta of their project for example;

{
  "pack": {
    "pack_format": 15,
    "description": "addon"
  },
  "features": {
    "enabled": [
      "builtin/quizia"
    ]
  }
}

Existing Flags

"builtin/quizia"
"builtin/space_travel”
"builtin/block_mixer"
"builtin/elevator"
"builtin/plushie"
"builtin/fabricator"
"builtin/player_sensor"
"builtin/time_control"
"builtin/scaling"
"builtin/chromatix"