openlayers - How to add key-value to feature

Question

in this example in openlayers webstie: here there is a style function the takes feature ans resolution as input parameters. my question is about the feature parameter. as stated in the code below:

style: function (feature, resolution) {
    switch (feature.get('layer')) {
      case 'water':
        return waterStyle;
      case 'roads':
        return roadStyle(feature);
      case 'buildings':
        return resolution < 10 ? buildingStyle : null;
      default:
        return null;
    }
  },
}),

the feature has a key named layer. and in this code:

const roadStyle = function (feature) {
const kind = feature.get('kind');
const railway = feature.get('railway');
const sort_key = feature.get('sort_key');

feature has other keys as shown. my question is, how these key were added to the feature, the code showed how the values can be retrieved given the keys, but how these keys were initially added?

i hope my explanation is clear