How do I make a connection between an entity and a component in an ECS?

Question

Recently I started learning about ECS and I decided to try and create a very basic implementation on my own. I made myself an enum with the component names and two basic components:

enum Components {
    VerticesComponent, ColorComponent
};

struct VerticesComponent {
    int id = Components::VerticesComponent;
    float vertices[];
};

struct ColorComponent {
    int id = Components::ColorComponent;
    float red;
    float green;
    float blue;
    float alpha;
};

Then I added a struct for an entity:

struct Entity {
    int id;
    std::vector<int> components;
};

And then I realised that my approach is flawed because I only add the index of a component to my entity but not the actual data stored in the component. So my question is: how can I properly connect an entity with a component?