As a junior dev, should I waste my times on Working on real world project or should I just solve leetcode questions all days long (interview questions in general)?

Which one is better for me as a learner?

Make a thing on your own.

Work on real world projects and on the side on your own projects.
Real world problems will make you more experienced because you have to deal with human errors. Solving riddles all day might help you to gain general understanding, but they can't simulate the quirks of real world systems intertwined with each other.

frankly, both. As both have their advantages and disadvantages.

Real work -- you face real-life problems and have to work out solutions to them, while minding the business needs, processes, limitations, budget, other real-life factors affecting your choices, mental health and performance. You'll learn how to operate IRL. HOWEVER, you'll be stuck with your stack, with your particular project, which means you'll be learning only that project-related things. That's a problem, because soon enough you'll want to use huge frameworks, which will be your BAU, so solve even the simplest problems.

Online challenges - will make sure you keep learning about all the domains, about how the algorithms work and their strengths/weaknesses, you'll learn how to manage concurrence, how to deal with data structures, how to build new ones, etc etc. You'll learn many things about the language of your choice and about programming in general. HOWEVER, you will not experience the real-life stress, distractors and limitations. You will not be solving real problems and you will not learn how to use frameworks, not to mention you will not know how to make a better solution to the online problem (you won't have a team to discuss with). And OFC you won't learn what teamwork is.

So I say take both the directions simultaneously.

If you start a project from scratch, you will learn fast how messy it becomes if you don't structure it well.

make one thing you can show and be proud of, then do leetcode questions all day while applying to places

Do the project on your own, think about some problems and try to solve them with your project. Best way to learn!