You can try codewars Algorithms - you start with beginner algorithms and then it gets more and more difficult the more you go forward
Algorithm challenges are great for honing your skills with the language, understanding of computer science concepts, ability to reason through a problem, etc., but you won’t learn much about software engineering by doing them.
If you want ideas for apps to build, you can just Google
<blah> app ideas (beginner app ideas, fun app ideas, etc.). I just Googled for app ideas and found this generator:
And if you don’t like any of its suggestions, you could try making a clone of the generator itself!
A common one is replicating common websites which tests your HTML/CSS/JS skills. A good one to start with might be Hacker News since it’s straightforward and you can copy some of the JSON responses from their API.