For the hover effect, you need to think about what that means. Generally, having something highlight on hover implies that when we click on the thing, something will happen. Typically that will either be navigating the user to a new page, or executing some kind of action (submitting a form, opening a modal dialog etc).
In this case you don't have to code that interaction itself, but you do need to have a clear idea in your head what you think it should do. In my mind, clicking on the image in this design would open up a bigger version in a modal. But navigating to a full detail page for the image would be an equally valid decision.
So now the important thing is what to do in the HTML to make this correct. If clicking on something navigates us to another page, that means we have to make it a link. So it needs to be wrapped in an anchor element <a href="/some-url"> . If clicking it causes some other action, that means it's a button, so it must be wrapped in <button>. If you don't do that, then the hover effect just becomes some meaningless decoration, which is not what we want on the web.
So then once you have the HTML right, for whatever you've decided is the appropriate behaviour, then you can do some styling. Whether you've chosen a button or a link, the CSS will be the same.
a.main-photo:hover {
/* some styles */
}
or
button.main-photo:hover {
/* some styles */
}
That will deal with the hovering. But, that's only for people that navigate with a mouse. Lots of people can't, or choose not to, so we need to handle keyboard navigation as well. For that we can use the :focus-visible pseudo class. Sometimes the hover effect and the focus effect should be different. Here I think it makes sense that it's the same, so your CSS would become
a.main-photo:hover, a.main-photo:focus-visible {
/* interaction styles */
}
