A good side project is like leetcode on steroids, so this is not very true.

Do you have some examples of good side projects that fits your description?

Leetcode specifically targets a specific problem (per entry). Side projects might contain one or two, but usually I'd guess it's a small part of the total worktime.

Exactly. There's no way I could work on enough side projects to cover all the algorithms and data structures and concepts that interviewers expect me to have at my ready.