how to make something change when th emouse hovers over it in css code example

Example 1: hover over something to make html visible

div {
    display: none;
}
    
a:hover + div {
    display: block;
}

Example 2: how to change another element on hover

// If the cube is directly inside the container:
#container:hover > #cube { background-color: yellow; }

// If cube is next to (after containers closing tag) the container:
#container:hover + #cube { background-color: yellow; }

// If the cube is somewhere inside the container:
#container:hover #cube { background-color: yellow; }

// If the cube is a sibling of the container:
#container:hover ~ #cube { background-color: yellow; }