Page cache is the entire rendered html output for a page. It’s useful for serving static content like a WordPress post.
Object cache is often the resource-heavy pieces that make up a page. For example, When you use WP_Query each result would be stored in object cache. This prevents WordPress from hitting the database every time WP_Query is used.
For example, if you use ajax for pagination, the next page of results would be stored in object cache and not in page cache.